It's completely possible, but usually animals from "bigger" lines, grow bigger than those from "smaller" lines. You also have to factor in cage temps, feeding schedule, prey size, etc. Also "not that big" is a relative term and I don't call my 7' Dumerils big. In a specific Dumerils conversation I'll say they're "bigger" but in terms of pythons/boas, I don't classify them as big. It's just a basis to go from, looking at the parents, that will give you a better idea than looking at a snake in a deli cup at a reptile show. Can the babies become larger or stay smaller? Absolutely!