Comparing Capacity

I am looking for a cup with a higher capacity than my small Lunette.   I had a particularly heavy cycle last month, and I filled up my small Lunette and nearly leaked through a regular pad in about four hours, and because of my job (student teacher), I don't have the time to empty it more often than that.  I know from the size charts that there are a few cups with higher capacities, but I'm wondering, how much difference does a few ml actually make?

Can anyone tell me from experience whether a larger capacity cup holds a lot more than a 20ml cup (small Lunette, small Yukki, small Fleurcup, large LadyCup)?

Also, I have a low cervix, so I don't think I could use anything longer than 52mm, which pretty much narrows it down to a large Lunette (24ml) or a large Fleurcup (29ml), but I'm in the US, so it would be hard to get a Fleurcup.

Thanks in advance!