The change to the size of blockchain data files The blkXXXXX.dat files in the new Bitcoin-Qt client (v0.8.1) are about 130MB each in size, while the old ones are about 1.9GB in size each. Does anyone know the motivation behind this change?
The reason is future pruning functionality.

Pruning will be implemented by simply deleting files (and related metadata in index databases) with only old blocks in them. Having smaller files means higher granularity in choosing what to keep or not.