Having an absolute number of an XP reward with increasingly higher requirements for each successive level already solves this problem without the need for a complicated % scaling-based-on-number-of-enemies-killed system.
To put another way, if you need 1000 xp to level up to level 2 and a kobold gives you 10, a kobold gives you 1% of the XP you need to level.
If you need an additional 2000 xp to level up to level 3, a kobold gives you only .5% of the XP you need to level.
If you need an additional 4000 xp to level up to level 4, a kobold gives you only .25% of the XP you need to level.
So on, and so forth, until at high levels the kobold effectively gives you 0% of the XP you need. (Note: the "increasing requirements" part is necessary, because if you always only need an additional 1000 xp, then of course the kobold will always give you the same benefit.)
This is what I never, ever, understood about the challenge rating system of 3e D&D. AD&D had already solved the problem of easy encounters providing trivial rewards to powerful characters, you didn't need to go out and add a whole complicated layer of abstraction on top of it.