It's an age thing. Dragons don't stop growing once they reach maturity, so the oldest ones can get incredibly huge. Blitz is about 220 years old (think 22 in human years) so he's still pretty small. Cherna is in her mid 400's so she's a pretty good average sized adult dragon. Kroft's in his 700's so he's the largest of the three. If Blitz survives long enough he'll grow to be as big as them someday.