Well, I had a look at my files and discovered that one of my vellum backgrounds had ended up over ten megabytes by itself, and you couldn't even see the effect I'd been aiming for! That's what happens when you play with noise and vellum.
Talk about file bloat! I redid the page in plain white and it miraculously shrank to a more suitable size. I resubmitted the kit and it hasn't bounced back yet, so hopefully I got it small enough. It'd better be, because it's way past my bedtime and I can't do it again...