Model Sarah Stage -- Super Skinny Pregnancy Leads to Big Baby!

The day after model Sarah Stage took a ton of flak for looking like the fittest pregnant woman in history ... she squeezed out a baby wayyy bigger than anyone with eyes would expect.

Stage gave birth Tuesday afternoon to James Hunter -- 8 lbs. 7 oz. and 22 inches long. Totally normal size for a newborn, and maybe even a little large ... considering Mom posted a pic Monday that looked like she'd just had a big lunch.

Stage's pregnancy was so easy, she even went out to dinner with pal Melissa Molinaro on Sunday night ... still looking like your typical rail thin L.A. chick.

Guys wanna get her pregnant, women wanna BE her pregnant.