Recreated a .umap and its less than 1/10th the original size?

Just further to this… looking back through perforce history on one of my larger maps… and the size jumps around a bit… at one point it was 280MB then it jumped down to 80MB and has been there for a while… I dont recall making any obvious reduction in map content to that extent at that stage… in fact between those two version i as actually adding stuff…?