Maybe the problem is that modern infants are getting fatter!

http://health.msn.com/pregnancykids/articlepage.aspx?cp-documentid=100142529&GT1=8404

It doesn't seem surprising to me that your child would grow at a slower rate
than babies fed the SAD foods.  It's like a calf eating natural pasture and
mother's milk compared to a calf force-fed corn and ballooning to an
enormous size quite quickly.

-Elizabeth