i have heard a common complaint from Many GMs about players having too much loot, too large a portable armory or leveling too quickly. but hear me out on my answer to all of these,
if your players are too wealthy or happen to be growing in power too quickly. it is entirely your fault as the game master or storyteller. this complain applies to any RPG. but let me pick on D20 for a bit. not because i strongly dislike the D20 system (i honestly do) but because D20 is an outdated format with many old problems.
in common D20 System Fantasy RPGs. there tends to be an Escalation of magical equipment as you progress. this magical equipment is often gained by means of being pried from the cold and dead fingers of slain NPCs of importance. you know who made those NPCs and allowed them to have the very magic items the players slew them for? You, the Game Master/Storyteller did. it comes from an old philosophy which i will detail below.
the moment you include a new Character Building Option for your NPCs, players are going to want to also be able to use that option for their own characters as well. so say in a medieval setting, owning a suit of plate, a horse or a plot of land, required you to be a noble. most player characters will try to find a way to become, slay or impersonate members of nobility so they can have the plate, the horse and the land. include a Jedi NPC with a lightsaber in a Star Wars game, and players are going to want to learn to utilize the force and wield lightsabers of their own.
everything you allow for your NPCs, will eventually fall into the possession of your players at some point. if you don't want your players to have something, please don't include an NPC that has it. it is better to say plate armor doesn't exist in the setting and stick with it than to restrict plate armor to the nobility.