On two WordPress sites I am facing the same problem. I exported the data as Rss (xml) files and imported them into new wordpress.com blogs as backup. Then, after upgrading my WordPress hosted blogs to 2.3.3, I exported the data from the wordpress.com blogs and am trying to import them in the hosted blogs.
But I get an error saying that the files are too large - one file is 4 MB plus and the other is 8 MB plus whereas the allowed limit is 2 MB!
Even the original exported files from my hosted blogs were that big (I have no idea how or why they could be so big) yet I was easily able to import them into the wordpress.com blogs (where the limit is 3 MB).
I am stuck now, with no backup. What do I do?