I'm not sure how they would prioritize orders either, but my assumption would be first in first out, as per any normal business operating procedure. Of course with items not in stock or on backorder that would naturally cause a later ship date. I don't think it has to do with size at all.
On that note, my order(22 items) was shipped today. Order was placed 12:59am 01/01/11. Got my order in a little late due to those website issues and trying to slim my order out lolol... not sure if I succeeded on that last part. I live in FL and it shipped from GA, so I think I should have it either tomorrow or the next day. Much, much sooner than I expected.