Yesterday I happened to go over every entry in the appSetting section of web.config. There is a key MediaFolderMaxSizePerFileInMegaBytes, which is set to 30 by default. That's awfully close to the cut-off value you're experiencing. I'd suggest overriding this with a larger value in user.config and see if it solves the problem. There are actually a number of related maximum size settings surrounding that in web.config, so you may want to experiment with those as well to get things working the way you need.
Jamie