This is amusing...as I work for corporate offices of GAP (which owns Old Navy) and we hear THE EXACT OPPOSITE of what you are all talking about from consumers!!
They complain that the ON fit is too SMALL and the shirts (t-shirt variety) are too short and too tight for those who don't wish to be baring the belly.
As a consumer there myself, I've always thought the main sizing issue is that it's not consistent. You can buy a size 8 pant in one style, then buy a size 6 in another style!
If you really want to let the Corporate honchos know what you think, go to our Gap.com site and link to customer service.
"My dreams, watching me said, one to the other...this life has let us down."