Faster than 828.539 mph?
Flying feral chickens. Boy does that bring back memories. The snickering was done by people named “Chris,” to be specific. 
Perhaps there were more moons. We know our moon was closer, and I think 3 can orbit 120 degrees apart w/o interfering w/ each other. (not that I beleive this theory)
I remember calculating that, for gravity to be cancelled out at the equator, the earth would have to spin at such a rate that a day would be around two hours instead of 24.
Also, I’m not sure I understand what GorillaMan is saying. How come the structure of the earth matters?
No, but the oxygen content of the atmosphere does somewhat explain the size of some dinosaurs relative to the size of their respiratory/circulatory systems. During the age of the dinosaurs, the oxygen content of the air was about 50% greater than it is now. Since the air had more oxygen, their respiratory systems had less work to do. Their lungs were smaller and less efficient; notice, too, that some dinosaurs back then appeared to have huge bodies but small heads with tiny nostrils. Less breathing issues…
Art
Probably that faster spin + non-solid Earth = planet more radically oblate, making for a much larger equatorial radius. However, in all those cases of superfast spin, the effect would ony be felt at the equator: at the higher latitudes you’d get just as much or even more gravity (closer to the center).
The entire Earth-Moon system is slowing down because of gravitational drag – the Moon already went into tidal lock with the larger planet. Because of conservation of angular momentum, this also means that the radius of the Moon’s orbit becomes larger over time.
I remember reading somewhere on the Straight Dope that, in addition to the big meteor that may have killed off the dinos, earth is being bombarded daily with a significant amount of meteorites. I think it was something on the order of several thousands pounds of matter actually making it through our atmosphere every day (though mostly in the form of harmless dust-mote-sized pieces).
If that’s true, then why would the mass of earth not be much greater now than it was 65 million years ago?
You have to realize just how insignificant several thousand pounds of matter is to the Earth. By my calculations, if the Earth’s mass was increased by 100 tons every day for 1000 times the age of the universe, it would see a 0.01% increase in mass overall.
Nitpick: But actually, the landing module leaves far more behind on the Moon than the rocks brought back. So each lunar mission has every so slightly increased the Moon’s mass and decreased Earth’s mass.
Another nitpick: it doesn’t matter if the meteor makes it through the atomsphere or not, it still adds to the mass of the Earth. The total mass of the Earth includes the mass of the atomsphere.
And to nitpick myself: yes, but those bits of meteor-vapor in the atomsphere don’t affect the gravitational acceleration of things on the surface. Spherical symmetry of the integrals, you know.
But wait there’s more: when calculating the gravitational force of the planet Earth on a distant object, we of course must include the mass of the atomsphere. But we also must include the mass of Moon, too. After all, the Moon has more than insignificant mass and is almost in the same place as the Earth, so it’s gravitational pull can’t be neglected.
Ok, that’s all. 
During the Jurassic the level of atmospheric carbon dioxide was also much higher than it is today - perhaps ten times larger. This would have had a strong effect on global warming, and both higher levels of carbon dioxide and higher temperatures would have increased plant productivity. Higher productivity means that much larger animals could support themselves than on an equivalent area today, and this may have been the key factor that permitted gigantism in dinosaurs during the Mesozoic.
Could you make the argument that weaker gravity would equal SMALLER animals, since weak gravity would cause weaker skeletons and less dense bone structure?
I thought I’d add something, 'cause I popped in here thinking the OP said something about the size of Dinosaur poop.
Carry on…
There is little evidence that dinosaur lungs were “less efficient” (than what…?), or too small for their body size, aside from a controvesial interpretation of photographs of a single species of dinosaur (Sinosauropteryx) by John Ruben, et al.. In addition, we have no good lung fossils anyway, so any claims as to the alleged inefficiency of dino-lungs amounts to little more than speculation, with little (if any) support.
Having said that, it is likely that the oxygen content of the atmosphere was as high as around 30% during the Mesozoic (compared to ~21% today), which may have made breathing easier for all concerned (recall that there were much more than dinosaurs hanging around during the Mesozoic).
To decrease the gravity of the Earth by increasing its diameter without increasing its mass, you would need to make the planet less dense; you could acheive this only by including vast subterranean caves (for which there is no evidence, of course.)
I am aware of the theories on varying oxygen and CO2 levels, and have posted on the subject before;
I believe the evidence for high CO2 levels is quite strong, and the evidence for high oxygen level less so;
oxygen level is pretty much limited to something near our level by oxidation and combustion, so that very high oxygen levels would not last long before this reactive element finds something to combine with, one way or another.
Forest fires are already difficult to control in our modern world, thank you very much!
Also I wonder about the nitrogen content of ancient atmospheres; the atmospheric content of this element also varies, due to nitogenising bateria in the soil; but how much, I wonder (somewhat off topic, I’m afraid).
There was recently a good article in Scientific American with some theories about factors that led to the very large theropods (T. rex et al.). It mentions things like continent-sized ranges and lack of competing predators.
http://www.sciam.com/article.cfm?chanID=sa004&articleID=00075F12-D550-1FF8-90AE83414B7F0000
That can’t be right. 100 tons (200,000 pounds) multiplied by 365 days in a year, multiplied by ten billion years (which is a conservative estimate for the age of the universe (most theories seem to favor 11 billion to 20 billion years old http://www.space.com/scienceastronomy/age_universe_030103.html ) equals 730,000,000,000,000,000 pounds. Multiply the age of the universe times 1,000, and you have 730,000,000,000,000,000,000 pounds. Off the top of my head, I have to believe that’s a lot more than a hundredth of earth’s total mass, which is about…
http://www.howstuffworks.com/question30.htm
Wow! I would have bet against Achernar’s answer and even spotted someone the 1,000X multiplier. Earth has a lot more mass than I would have guessed. Achernar is right. It looks as if Earth’s mass is about 13,200,000,000,000,000,000,000,000 pounds, a hundredth of which equals 132,000,000,000,000,000,000,000 pounds and that’s about 180 times the 730,000,000,000,000,000 pounds suggested by Achernar (with my conservative placing of the universe at ten billion-years old). If I put the age of the universe more in line with average estimates, then Achernar’s answer is right on. Staggering numbers. 'Sorry for not using scientific notation. But it’s comforting to know that if Earth has been been amassing 10,000 pounds per day for 65 million years, then it’s only accumulated a “miniscule” 237,250,000,000,000 pounds.
Not “only by including vast subterranean caves.”
The average density of the Earth is twice that of the rock at its surface. Decrease that density, and viola you have an increase in diameter without an increase in mass.
OTOH, on a different board, we’ve been discussing this issue, because it came up in relation to the Expanding Earth theory of Carey.
Doesn’t Earth give off a good amount of gas that is in its highest level of atmosphere to space? Our atmosphere isn’t exactly locked onto the Earth and it seems inconcieveable that some molecules wouldn’t escape.
Though I guess Earth would pick a lot of it up again the next trip around the Sun.
There are actually three ways to change the gravity of the Earth. As already pointed out, you can change the mass or radius of the Earth, but if you changed the gravitational constant, that would also do the trick. In fact, there have been some speculations that the gravitational constant might change over the evolution of the Universe, but these speculations have already been discounted, in part by evidence from the fossil record. Changes in gravity would have resulted in extreme changes to the luminosity of the Sun, with the result that the Earth would have been uninhabitable.
Ted Holden’s still around, complete with feral chickens, though they don’t seem to get quite the same emphasis these days. Nor does Splifford the Bat, alas.