Though neither Xizor nor Jabba were by any means young, both had just been handed the reins of power established by their respective criminal families.
Jabba was no doubt a formidable figure in Galactic mob politics, but his empire had a great deal of provinence, even before he took the helm of it.
The Hutt families are a millennia-old powerhouse in the galaxy, controlling vast economic resources and employing sizeable mercenary forces. Hutt Space is a large chunk of the Galaxy that remains more or less independent throughout much of Galactic history. Jabba advanced to a position of power within his family before he started consolidating his criminal empire. His career would look something like this:
Born into top tier of powerful Hutt family.
Establishes a reputation as daring, enterprising and generally proactive through his independent gun-running business.
Impressing his family, rises through the ranks of the Cartel (most likely in a bloody and untoward Fashion).
Eventually reaches the top as head of the Cartel.
Consolidates power during the rise of Palpatine and the Empire by adapting and cutting deals with the new powers-that-be.
Jabba's climb to power is still impressive, but he it is not an entirely self-propelled journey from independent freighter pilot to Head of All Crime in the Galaxy.
Edit: also, what the other guys said.
