Kilts are flattering on big guys because of the cut. Well, as long as they're generally worn above the gut. Wearing your kilt (or jeans, for that matter) with your belly hanging over it is unsightly.

Being a widebody, I know from whence I speak. My ego is stroked more from looking good in what I'm wearing than squeezing into something I wore in high school so I can say, "Gasp... wheeze... yup. Still fits!".

As far as skinny guys... well, I've seen some real beanpoles in kilts. I think it gives some bulk that looks good. I could see a problem if you have really, really skinny legs, but why let that stop you.