yeah..it's because the characters are shaped differently..I've been finally going through my archives to get stuff into poser..hair and hats, and really bulky clothing (which is actually pretty rare) can be made to fit other characters, but most of the Mikes are really 'cut', so the normal default Poser (dork) clothes won't cover his chest.
There's a program called 'the Tailor' that can alter some clothes to fit other characters. It's not free, but I've heard a lot of good things about it.
When you get a character, start in Freestuff, and go to Poser, then go by category..default characters will have a lot of stuff, but the biggies are Vicky and Mike, from http://www.daz3d.com. They're not free either. Vicky has a lot of clothes that are good too.

I wish I'd said that.. The Staircase Wit

anahl nathrak uth vas betude doth yel dyenvey..;)