Kate Middleton -- Not All Famous Pregnant Chicks Have to Be HUGE

If pregnant women were capable of jealousy, one would say Kim Kardashian and Jessica Simpson might be a little covetous of Kate Middleton's physique -- 'cause they're all due around the same time, and well, one of these things is not like the other.

The Duchess of Cambridge visited the Willows Primary School in Manchester, England yesterday ... and you could BARELY tell she was with fetus.

Kate's due to give birth to Prince William's baby in July ... the same time Kim K's due ... and Jessica is also expected to deliver kid #2 sometime around then.