Just the facts, man:
MINILLA: This little guy was NOT human sized when born, but stood 13 meters in height. He only grew to 18 meters at his tallest, and this was due to eating fruit on Solgell Island, natural growth, and perhaps absorbing residual radiation from Godzilla.
LITTLE GODZILLA: Its ALL about the radiation! Baby started out at 1.64 meters and somehow balooned up to 30 meters in less than a year. This is due to two things: first, his proximity to Godzilla and the radioactivity he naturally releases; and second, Birth Island has a number of underground radioactive uranium deposits, which is what attracted Godzilla and Baby there in the first place. The island provides energy/food for both creatures, and its possible the excess radiation abosrbed by Little Godzilla stunted its natural growth, giving the little monster its bizarre (but super cute) appearence.
You're welcome! ;)