That is not the actual official bootstrap.dat. That is the file produced by a third party and has also been compressed from 50+ Gb to around 35+. The smaller size makes it faster to download (duh). I was referring to the official bootstrap.dat which is now obsolete and is still the full size of the blockchain, not compressed.
It's a standard zip compressed
bootstrap.dat file, combining all blk*.dat files out of the blocks directory.
Maybe we should discuss adding a zip level compression (or better suitable) algorithm to the Bitcoin-Qt client to reduce the amout of data needed to be transferred to update the block chain.