I've had it both ways. I do find that leos by the age of 2 have most likely reached the approximate "final" weight, though I don't know if that applies to giants.
My geckos seem to reach full length at 2 years old. Most will continue to gain weight slowly after that, and increasing in girth. Their weight max out and remain stable at around 3-4 years old.
e.g. One of my male, Hardee, is a 4 year old that is on the slim side. He stayed 78-80g since he was 2 years old. But he began to grow again this year (horizontally). I weighed him last week and he is 90g.