Model Sarah Stage -- See, My BIG Baby Had Plenty of Womb (PHOTO)

People don't want to believe model Sarah Stage gave birth to an 8 lb. baby -- especially since her belly only looked like it had a quarter pounder with cheese in it -- but here's photo proof of the big guy.

Sarah's son, James, is looking totally comfy and snug after his arrival on Tuesday. As we first told you ... James weighed in at 8 lbs. 7 oz, and 22 inches ... all above average measurements for newborns.

Sarah stirred controversy, disbelief, and probably some hatred -- when she posted selfies looking super skinny for any woman ... much less one who was 8-9 months pregnant.

We get the feeling she didn't mind the heat -- at least based on that lingerie pic she posted ... just HOURS before she officially attained MILF status.