There are a lot of potential toggles that can help you create a giant baby (or distinguish between an elephant and a dragon of the same size).
While level tends to increase attacks bonuses (THAC0), you can also have proficiency and attribute bonuses, as well as size modifiers. In 3.x, a giant baby might have relatively high hit dice, but would not have proficiency with weapons, and would have a fairly hefty size penalty; if the attributes didn't give a major bonus, it'd work out pretty well. Whether it makes sense for a Giant Baby to have a STR of 10 is one you have to answer - I think it does even though that might be unusual for other creatures of that size, it might reflect the reality of 'uncoordinated attacks' better than a 25 or 30.
I do think that in the abstract it might make sense to have bonus hit points for size that are independent of level. Ie, a baby might be a 1/2 HD creature, and be Tiny. For every size you increase it, you could add extra hit points - something like +4, +8, +12, +16, +32, so going from Tiny to Medium (2 sizes) would add 12 hit points total; going to Huge (4 sizes) would add +40 hit points.