Forgive the click-bait nature of the title, but after having a number of discussions with friends and friends of friends about XP rewards in games, and seeing umpteen million discussions about XP rewards online over the history of GM and table top RPG advice, I figured it might be worth it. XP is a toouchy subject for a lot of GMs. There is a lot of confusion over what is, in actuality, a fairly simple mechanic to represent player character growth. Now, like most things in RPGs the abstract nature of XP makes it weird at times, but that confusion is mostly self inflicted.
With all that in mind, here are some reasons why you, or someone you know, may be doing XP rewards in their game all wrong.
You're Too Worried About The PCs Getting Too Big Too Fast
First off, disclaimer, I think all fears about PCs getting "too powerful" are suspect at best. There's no such thing as "too powerful" for a PC. There may be "too powerful for this story idea" but that is not a problem with the PCs power levels, it's a problem with the story. You don't give Superman a "lone scared gunman is robbing a Ma & Pa liquor store on 5th and Bleaker" story and expect it to take more than a fraction of a round to resolve, and anyone who tried to make that story work would have to bust their ass to make it work with any number of other pre-established situation modifiers (red sun, kryptonite everywhere, the gunman is actually Batman). All of these change the story. This doesn't mean that Superman is to blame for being too powerful. It's on the story teller (the writer forcomics, and the GM for RPGs) to come up with a situation that challenges Superman in interesting ways.
So if by extension of my argument that you shouldn't be worried about PCs getting strong fast you deduce I'm saying you can't give too much XP, where is the problem? Well, the problem is you might not be giving out enough XP. Think about it. One of the universal goals of players (yes, there are some exceptions to this) is for their character to progress and grow stronger. XP is the mechanic in the game for them to do this. So, what happens when a player's character isn't progressing fast enough for them? Well, they get bored. They lose interest. They stop caring about their character - and for many players that means they stop caring about the game. If you're lucky, they bring in someone new or they leave the game. If you're unlucky, when they leave the game other people leave with them. Why? Because you weren't feeding their need to feel like they were progressing.
You Don't Reward The Right Things
I can't tell you how many times I've seen a GM complain that their players solve every problem with violence.(and in some cases every is literal, the PCs do things like kill shop keeps instead of paying for goods) The PCs just go around and murder anyone who has something they want, or who gets in their way. You have the King tell them they can't travel across the magical garden to the wizard's tower where their next quest is, and guess what? You now have a dead king and half the kingdom is on fire. The funny part is, frequently (especially for D&D and D&D-clone GMs) when asked what they give out XP for the GMs answer that they only give out XP (or full XP) for killing the monster. Meaning if the PCs negotiate their way past the problem, bribe, use trickery, or otherwise get around the monsters/obstacle without violence they don't get XP.
See the problem? This isn't complex psychology, it's a simple pavlovian response. The PCs want XP, and to get XP they have to kill. In fact, the only way to get XP is to kill. So now if the PCs want to progress (what XP lets them do) they have been taught that they need to kill. Sure, other solutions may be around, but those solutions not only cost time and effort to pull off, but they don't get XP for it. So killing their way through the King and his guards? +900 XP. Negotiating past the King and his guards? Time, effort, and 0 XP. So that is lost time, lost energy, and lost XP.
You Don't Have A Plan
This stems back to the common issue of "PCs get too strong too fast" which I don't feel is a problem, but does highlight another issue for that GM. As the person who gives out the XP, and as the person responsible for approving what is allowed in your game, the onus is on you to know what your players are capable of and to plan accordingly. How much XP your players have, and how much they have been getting, should not be a surprise to you.
Every game I've seen has suggestions in the GM section for how much XP should be given for hours of play, number of sessions, and story breaks. Some go into detail for all three, others are more vague, but they have examples. These are good numbers, and they're based around solving the above 2 problems. You give X amount of XP for every Y hours of game play with the idea that after Z sessions for players will have acquired Q amount of XP and progressed R distance along their various power curves.
If you don't have a personal version of this plan, make one. Use it when planning your session.
XP is a powerful tool to control your game. Use it right and your game gets better. Use it wrong, and the game can literally fall apart. Be careful, have a plan, and execute upon it.