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 millenia-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 Untowards 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.