Companion leveling was adjusted back in r68 by issue 3645. I'm not sure what the exact mechanics are now, but the effect seems to be that it is either impossible or extremely impractical to level up companions past a certain point using weaker monsters. While this is perhaps more logical, the problem is that it takes a lot of levels to make a companion powerful, unless the base monster type is very good. With most companions it's almost impossible to gain enough levels, because they will just die to the stronger monsters required to level them up.

The nerf should be reverted at least partially. If pre-r68 companions were too powerful, any nerfs to that leveling should be targeted at the high end, say level 50+. But before that point it really did not need to be any worse, and the change has effectively killed the companion playstyle apart from scrolls of familiar summoning.