Let me pick this subject to have a first post here. This might have been mentioned before, but it took me a long time figuring it out. That might be just me, but I doubt it:
When changing from LR3.x to LR4 my catalog size increased considerably. This coincided with a long photographic trip to the US Southwest and many images in dusty places. Long story short, my catalog exploded from 350K to over 6GB. For 35K images in total. This not only slows down performance when working, but also slows down and fills up the backup chain. I tried everything from optimizing the catalog to rebuilding it from scratch and doing some fancy stuff on the database directly with SQlite commands. Nothing helped. But deleting the history did help. A lot. The catalog went from 6GB to 700MB. That is a massive reduction in size, much more than I had expected from the information I was able to find on this subject.
Why did the catalog grow out of proportion? Probably because I do a lot of local adjustments and I had to deal with thousands of dust spots in my SW images. And when you are working on multirow, multi exposure panoramas, that all adds up. I print some images really big, so every spot has to go.
So, if you do not need all that history, it might be a good idea to delete it once in a while.