I've seen slab allocation schemes (not sure if that's a standard term for it) work in scenarios like inverted indicies where you want to keep your datum key size small (32 vs 64bit).

The intuition is that you don't need granular memory addressing for storing large chunks. You only need to have addressability at the min(sizeof(doc)) level. So you can usually store the key in a 32bit int or less.

More detail can be found here: https://gist.github.com/1947190