Huh, that's not how I understood it.
What I'm talking about is the overall number of enchanted hits your weapon can hold. I thought that you determined how many hits your weapon held based on how much damage each hit did. I noticed that when I made my shock mace, I would get 25 or so hits with +12 damage, but 100 or so hits with +6 damage.
And then, as you said, the larger the soul gem you use the more hits get charged. So if you have a +12 shock mace, it will get filled up with a petty gem because you only get ~25 hits with it before it needs to recharge. But if you have a +1 shock mace, you'll need several petty gems to recharge because you get ~300 hits.
I'll take a closer look next time.
EDIT: take a look at this and hopefully it will clarify what I'm trying to say:
http://www.youtube.com/watch?v=pL1nvf0It_E
Fast forward to 3:40 and he is frost enchanting a sword, you see the slider menu come up. At +9 damage it gets 97 hits before a recharge, and at +10 it gets 86 hits before a recharge. Basically, you determine how many charges a weapon has while enchanting it, but there are payoffs/drawbacks for more charges/more damage. (i.e. at +8 it would have 106 hits, at +7 it would have 115 hits, etc).