true.. but i like mine jerseys bit baggy under the arms as i dont like to be restricted when i use the drops on my road bike. Personal choice really i like my tackle not to move around in my shorts too much either so i go for a snugger fit downstairs.
Sometimes you find different makes of jersey/top have different sizes. Adidas for example, seem to make all their stuff one size bigger than the label says, while Nalini make them 1 smaller.
Trail and error really.