I’m thinking in geological time frame.
Dogs go from Chihuahua to wolf.
Cats from tabby to lion.
Mankind from Lucy to Masai
Snakes?
Whales?
I think sloths would be up there too. There are some extinct ones that were really enormous. As another example I know that there were some forms of elephants found on islands that were quite small.
The problem though is how you define kinds of animals. If you make your categories sufficiently broad you can cover quite a range of sizes.
Horses started out as eohippus, which was about the size of a small dog, and now can grow up to over 2,000 lbs. for a big draft horse. I would think that’s gotta be up there.
Fish go from litle micro-guppies up to whale sharks.
Jellyfish?
(bolding mine)
More like chihuahua to mastiff. But I think fish is going to win this bout.
Well, for that matter mammals range from pygmy shrews (or, apparently, bumblebee bats) up to blue whales.
Eukaryotes cover an even broader range.
Puffer fish. Duh.
What?
Snakes range from the Burmese Python which are proven to grow up to 400 pounds to the threadsnakes that can weigh less than a gram. A 992 pound snake was reported in Indonesia in 2003 but I can’t find confirmation that it wasn’t a embellishment.
That leaves us with a known differential of somewhere between 181,000 times size at the conservative end and closer to 700,000 if you pick odd examples.
You need to define the size of your “group” for this to have any meaning. Already we have posts in this thread using group to mean everything from a single species to the entire eukaryotic realm.
A sense of proportion might help.
I mean, the largest spider on Earth isn’t all that big, but it’s infinitely larger than rhe world’s smallest spiders. The difference between the biggest and smallest spiders must be FAR greater than the difference between a Chihuahua and an IRish wolfhound.
I was going to suggest family as a taxonomic level for which this is a reasonable question. Off the top of my head, mustelids go from the least weasel up to the sea otter and wolverine.
I might also discount domestic animals like the dog which has been artificially bred up or down in size by humans. That said, the dog family still has a notable range. The smallest canid is the fennec, which weighs “up to 1.5 kg”, about on a par with the size of a chihuahua:
http://en.wikipedia.org/wiki/Fennec
cute little thing.
I think we should stick to one species, in which case, the smallest dog is maybe a few pounds fully grown, and up to…what, two hundred for a mastif? Three hundred for a really fat one? Now, I imagine the difference betwen the smallest and largest blue whale is probably much more than that, but as a percentage of the weight of the smallest sample of the species in question, the domesticated dog might very well win. How many other species can have the largest member weight a hundred times as much as the smallest member? (Fully grown.)
Back when I was in high school (about 10 years ago) I read about an elephant called the Congo Dwarf, which never reaches over 5’ in height. Since I love elephants and am barely over 5’ in height myself, I’ve used it as a name ever since. However, I can’t find the book which had the information so I can’t back it up.
It depends how you define “animal.”
Can we lump all insects together? Some are thousands of times the size of others.
This subject should be measurable somehow.
Comparing a cat to a lion would be the same as comparing a tiny microscopic fish to a whale.
An interesting spin off of this question would be what is the greatest percentage differential in size can like-animals still mate and create off spring. Not sounding sick but if you don’t throw that off spring word in there one can consider the sick men and women out there who induldge in zooskool.
Can a clydesdale mate with a miniture horse? Can a great dane mate with a chiwawa? I would imagine a Great Dane has to be 20X larger then a chiwawa.
Primate wise-Squirl monkeys cannot mate with Gorilas, nether can domestic cats and lions, nor fish and whales. Wow, this can go on and on without measurable parameters.
This web site is fascinating. There is a subject on everything.
There are some species with major sexual dimorphism: Gorilla gorilla* males can have twice the mass of females. “One of the most extreme examples of sexual dimorphism is found in small worms of the genus Osedax, which live on whale falls. The females feed on whale bones. The males live inside the females and do not develop past their larval stage except to produce large amounts of sperm.” Fortunately, sexual dimorphism is relatively small in humans; male and female height/weight ranges mostly overlap.
*I just like saying “Gorilla gorilla.” 
I think that “fish” is too broad a category. But if you look specifically at sharks, I think we have a winner.
Dogfish are only about 2 ft long, and whale sharks are the biggest fish in the sea, up to 46 ft long.
When it comes to mammals, I would have to vote for the kangaroo, even over the blue whale comparatively speaking - there’s an amazing size difference between newborn and adult.