EDITED UPON RECEIPT OF SIZE M:Edit #2: received grey medium, it's significantly smaller than the pink medium - see added picture with measuring tape. I exchanged, assuming I'd been sent a small, but no, the sizing is just extremely off. Finn is OK in it, but it's not as good as the pink. Too bad. The pink is also softer.First off, my boys look dapper AF! Finn (grey sweater) is approx 12 lbs, 15 inches long (if not at all stretched out) and lithe as a weasel. I ordered a L upon reading the other reviews. He looks great, but it's a bit bunchy under the arms and bit too long. For testing, I tried the grey L on my smaller cat, Sawyer, who is approx 10 lbs and 12 " long. It is certainly big on him, although he seemed quite comfortable. I ordered a M in pink for him and think the size is a better choice for them both.The large: Finn doesn't hate it, but he does pull out of it quickly around the house, he doesn't in the Medium, both cats would wear the Medium all day without complaint. I have yet to try it with the harness, though, which may make a difference.It's shockingly easy to put on. When I took it out of the bag I expected I wouldn't be able to use it due to the effort it would take - nope. Finn basically did it himself, Sawyer took a little coaxing due to inner issues of being restrained, but it was painless and once on, they were both happy. No clue how they engineered that, but well done! I really thought that Large was correct on my bigger cat, but I will be swapping it for a Medium, having tried them both. There was just too much bulk for him to want to keep it on.This is not warm. If you have an animal in desperate need of warmth, get something else. This just adds a little insulation and a LOT of charm. (I swapped out the pink sweater button because that plastic "hammered rose gold" thing was silly, to me)Chris Evans, eat your heart out ;)