Why are the middle planets (Jupiter and Saturn) so big and the outer plants (pluto and mercury) so small?
The Solar System formed from a cloud of gas (hydrogen and helium) and dust (a term which includes any solid particles: bits of rock, bits of metal, ice crystals, etc.)
The clould collapsed, which essentially means that more stuff ended up at the middle. Today, we call the central concentration of mass “the Sun” 
but even before it was a proper star (that is, before it started burning hydrogen), it was still warming up, due to the energy released by stuff falling in.
This made the inner parts of the nebula wamer than the outer parts. In fact, close to the Sun it was warm enough to melt the ice crystals. Thus, as the planets began to form, in the inner nebula, there was only metal and rock to build 'em. This is where the terrestrial planets, Mercury, Venus, Earth, and Mars, formed. (And the asteroids, too, but the reason they’re small is that Jupiter’s gravity stirs them up so that when the smack into each other, instead of sticking to build a larger planet, they have a tendency to smash each other into smaller pieces.)
Farther out in the Solar System, there were little solid bits of metal, rock, and ice. Because of the extra raw material available, planets could grow larger. In fact, their cores of ice and rock got so big that they had enough gravity to hold on to hydrogen and helium gas from the nebula, allowing them to grow to gargantuan proportions. This is how the jovian planets, a.ka. the gas giants, Jupiter, Saturn, Uranus and Neptune, formed.
Now, remember how there was more stuff toward the center of the cloud than in the outer part? The inner part of the nebula was denser than the outer part. So even though there was more frozen stuff in the distant outer part of the Solar System, even when you add up all the different raw materials, there was still less stuff overall. This is where the small, icy rocky bodies of the outer Solar System, Pluto, the Kuiper Belt objects, and the Oort Cloud bodies, formed.
In summary, in the inner part of the Solar System, there wasn’t any ice, so the planets are small. In the middle part, where there was ice and rock and metal, and lots of it, planets are huge. In the outer part, the nebula was thinner, so once again the planets are small.
Couple of questions about that.
-
IIRC, gas giants are believed to orbit other stars at much smaller distances than 1 AU. I wouldn’t know if this is the exception or the rule, but given our observation methods we’d have a much easier time detecting jovians that are very large, very close to the sun, and have a ridiculously short “year.” Does this raise an argument against the ice theory?
-
Pluto is not a planet. It’s a bit of space junk, and there’s lots of other crap out there like it. If Pluto’s a planet, then kids are going to have to memorize a very long list of “planets.” And that would be bad.
To point 1: I think the current thought is that those big planets formed further out, but they got pulled in close to the star over time (probably destroying any planets viable for life on its way).
To point 2: ‘Planet’ is as scientific a term as ‘continent.’ Pluto is just a planet by tradition and common agreement, just as Europe and Asia are separate continents, even though they clearly aren’t separate pieces of land. Because there’s no formal scientific definition of ‘planet,’ arguing about it isn’t even moot.
Well, no one REALLY knows the composition of those planets, their solar systems, or their suns. There could be some odd stuff going on.
And why you gotta be a Pluto-hata?
For the record, it is hazy, and depends which definition of planet you use. Even the dictionaries cover all their bases.
So you’re content with the size of Uranus?
You know, if the Chinese found a new planet and named it Pei Nas or something, comedy would COLLAPSE INTO ITSELF.
To point one: Yes, that’s a problem. As bup says, maybe the planets formed far away and migrated in, or maybe our understanding of the way planets form is flawed. After all, for the longest time we’ve only had the one solar system to study.
To point two: I say Pluto’s a planet. 
:Thbbbbbttt:
It remains the largest known Kuiper Belt object. If and when we find one of comperable size or large, then I’ll stop calling Pluto a planet and start calling it “The longest known, nearest, best-studied, and therefore most interesting and important KBO.” 
(Seriously, this is such a nonissue in astronomy . . . The only people who get excited about it are laypeople and that one guy at the Hayden planetarium.)
As far as these large planets being gas giants, I should think that the black body temperature of a planet so close to a star would be high enough to rapidly drive hydrogen and helium from the atmosphere. Even if these monsters had formed further out, once drawn in, they should quickly be reduced to their nonvolatile core material ?
Squink, there are two factors that determine the rate of escape of gasses from the atmosphere, temperature and escape velocity.
The “roasters” or “hot Jupiters”, as they are called, are likely to have lost some of their mass due to the escape of hydrogen and helium (i.e. they’ll probably be further from solar abundances than Jupiter). However, because they have a strong gravitational pull, they can still retain most of their bulk.
Observation of sodium lines from of an extrasolar planet that passes between us and its sun confirm that it has a gaseous atmosphere. AFAIK, we are still waiting for a detailed spectrum.