The fattest champion in league is more mobile than the swift scout

Fat contains a lot of energy, where as a hamster does not.

Simple answer: Inertia.

Teemo has very small mass and therefor has to keep exerting energy to keep his momentum from decaying. Gragas with enough initial force can keep moving for much longer without reintroducing energy to keep himself moving.

Teemo moves with very small, fast steps, which is why he has very high speed. Gragas on the other hand Lurches around relying on swinging his massive weight to propel himself in short, explosive bursts, in between which he is quite slow and sluggish.

Well what would he give up for the mob....oh wait trade-offs don't exist anymore apparently. (I understand counter play exists but mobility should have a price where as these three have little to none for thiers)

The last thing we need is more Teemo buffs. Did you see the ones on PBE?