Can someone who’s read the source material give us the backstory on Dragzilla?

I haven't read the books but dragons get bigger as they get older.

Vhagar is around 180 years old and dragons growing their whole life (most of course in the first years). Meanwhile Arrax is same age as Luce, around 12 years old by show time.
Balerion is considered to be the biggest dragon (at least of post-Valyria) and you can see his head several times during the first season. (https://static.wikia.nocookie.net/gameofthrones/images/5/50/BalerionHotDGuide.jpg/revision/latest?cb=20220823031421)

She was mounted by Visenya, one of Aegon's sisters when they conquered Westeros

https://youtu.be/wJ3iBPsd6mc
Comparison of dragons by size