Old Bitcoin's blockchain format took up more space, I believe after skimming through my Roaming folder. blk1-4 + index in Bitcoin folder = 8.6gB. blk1-5 in "Blocks" folder shows 6.79gB. An additional 879mB is in rev.dat files, 32mB in Bitcoin->blocks->index folder, and 200mB in Bitcoin->chainstate folder. 16.6gB total, but I believe that's because I didn't do a truly clean install. Perhaps you have some of the old mixing with the new?

(sorry to thread-jack -- can I delete blk1-4 + blkindex in Bitcoin folder? I installed .8 on top of .6 without deleting that folder. SSD's down to 12.6gB free space, so removing waste would be nice.)