I like this idea... But I think 15mb might be a bit better for the sake of making sure these small filesize maps aren't just "lag free hangout" boxes with a mirror.

As someone with relatively slow internet I do love this idea. On the flip side I feel like this would really push players to make nearly empty worlds and create a negative stereotype against <10mb worlds. I personally think having the opposite, a 50, 100, or 200mb world or greater category would be more useful.

This sounds like a fun way to challenge optimization techniques. Care would need to be taken to avoid it taking prevalence over all other categories and becoming the only way to have a popular/active world.