I guess I've always thought that nearly every baby (especially breast fed!) gets nice and chubby with rolls! I know VERY FEW EBF babies who are less than the 50th percentile (despite the WHO's charts saying EBF babies are generally smaller!!)
My nephew is EBF and at 7mos old he is outgrown 9mos clothes and only fits in 12mos. He's been soo chubby since his first month (he wasn't a big baby though).

When did your baby get chubby? Because my son is 9 weeks old and just starting to outgrow his newborn stuff, and some still fit so well. He is about 11.5 lbs, and doesn't seem chubby to me at all, but he is a champion nurser and eats a lot.