Well, I think from what I've seen is it mostly cut, esp in pants as they get older. Boys are bigger and boxier. The other thing I've seen is that especially winter play-type pants are often thicker for girls than for boys. I'm thinking specifically of the Kohl's Jumping Beans brand. They have pants that are elastic waited and wide cuff, but are similar to sweat pants in material and thickness. I LOVE these for DS1 in the winter, but I found that the boy's version wasn't as thick. So I bought DS girls pants and cut the pink string/bow off. Frankly, they fit him better because he tends to run small in pants anyway.
DS - Adopted by loving parents 1995
DS1 7/2009 ('Stachio)
DS2 9/2011 (Peanut)
I *just* weeded through all of my boy's clothes for a friend due next month with a son.
I was genuinely surprised with how much I thought could go both ways. I ended up keeping a ton of onesies, sleepers, blankets, etc. There were also a few sleepers I kept no matter how 'boyish' they looked. I distinctly remember both of my newborns snuggled up in them and new baby will be too, I'll just toss a blue bow on her head to match if we are going anywhere.
I also kept all of the jeans (along with all the cotton pants in any color but blue) and all of the PJs. I really dislike all the embroidery all over girls jeans and I thought the cut probably won't matter a whole lot with a diaper on anyway. As for the PJs, I'm just too darn cheap to give away perfectly good PJs just because of the styles. (we don't really do character clothing anyway) Monkeys can go both ways as far as I'm concerned.
Super Crunchy Mama to:
J, my lawyer in training '06
Tsunami Sam '09
& sweet baby A '12