Now I understand - and I agree that the jump in size of Google's index probably has something to do with bigger challenges in getting rankings. However, those graphs are for the number of individual URLs, and it's simply true that Google indexes more individual URLs from a domain. I don't think that means more domains are in competition.
All that doesn't mean that the number of pages indexed has something to do with Panda. Panda is clearly quite surgical - it affects some sites dramatically and not others not at all, and the size of the site doesn't seem to be a factor. Also Panda was in development for more than a year, long before that spike in indexing.
At the same time, members here have noticed very intense crawling from googlebot, just before each major Panda version was set loose. Maybe there is some connection I'm not seeing.