For the most part, XP is tied very strongly to real life. For instance I started an exercise regime a few months ago cause frankly I needed it, I have no upper body strength and could barely do 10 push ups without resting yet after a few weeks of doing just 10 push ups per set, I eventually got to the point where I could do it easily. On the other hand, I could do 10 push ups every day for a whole year, but it wouldn't mean I would be able to do 100 push ups in one setting. I need to build myself up cause the repetition (grinding) got me strong enough to do it easily, but without really upping my game I eventually reached a point of getting diminished returns cause I needed to do more push ups to increase my strength and stamina. Likewise in game, as you level up, the XP you get is diminished, as VeloZer0 pointed out, most do this by making the requirement for the next level to increase while others like Suikoden or SRPGs make weaker enemies give smaller returns of XP.

In terms of games that have really expanded XP gain, Xenoblade is probably the best example in recent years, you get XP for killing enemies, discovering new locations, completing quests, but you also get them for collecting certain items or reaching ceretain achievements like killing your 100,000th enemy, or completing a characters skill tree, or finding your tenth location, or meeting your 50th named NPC. It actually takes the silly bragging rights achievement system and gives it a practical use within the game and I really wish more games would do that.