I did a 1:13 Quicktime reel in Sorenson Pro 3 (2-pass VBR) that weighed in at 5 MB. It was 15 fps, 320×240, set to a data rate of around 75 KB/sec.
By my reckoning, your 320×240 should be around 7 MB if you were to use my settings with Sorenson Pro 3. Gotta buy that codec. 🙂
Try my settings in your codec, but beware — users require QT7 to play back H.264, if I recall.
(by the way, do you mean megabytes of megabits? MB is megabytes, Mb is megabits)
(mb is microbits, but nobody’s liable to confuse that)
Steve