I see Thompson has the Poseidon which looks great size-wise.  How come they're the only ones that offer anything that small?  

Everyone's pistol cans look as long as their rifle cans.

Because micro cans like the Poseidon must be shot wet and wiped for decent suppression, and they get extremely loud after 3-4 shots. The market for these types of micro cans is extraordinarily small since most folks want something as quiet and low maintenance as possible.

First, the larger diameter hole in the front (.50" in case of a .45 can) the can must be of a larger volume for adequate performance.  That combines with the need for a Nielsen device (booster) which takes up about the rear quarter of the suppressor.  Compact cans are a niche market.  You can't compare them to full-up pistol cans in terms of performance or reliability.  The Griffin Revolution gives you the option with the same tax stamp of having either a short or long suppressor.

I purchased a micro can - the Poseideon in fact - but realistically the market is small, so it doesn't pay for the manufactures to get involved with them.  You're dealing with the mess of a wet can, the mild aggravation of dealing with wipes, and if you look online there's a test of the Poseideon along with another really small can...and the suppression isn't great.  If you understand that the Mazda Miata of silencers isn't going to do what a mini-van will, great...but too many people will/would bitch about them to make it worth while.  I went in with my eyes open, and already owning a Gem-Tech Multimount.