As people have said i don't think the bump is an indiactor of the baby size. Some people just manage to conceal them well.
My GF and i are generally about the same size, she looked like she'd had a big dinner and gave birth to a 7lb something baby, i on the otherhand was the sizre of a small whale and gave birth to a 5lb 14 baby.
I think you're just lucky to have one of those compact bumps.