The size difference between the EO carts and the kiss carts is minor maybe half an inch, its literally just a length of the drip tip, so a minor size increase for a major capacity upgrade.
As for the amount a kiss can hold it would be about the same as any of the exposed coil type heaters you've seen, The instructions say you can add up to .3, but it makes it very very messy, leaky, and you just end up scooping all the oil back up onto the coil anyway.
edit: Also mixchu id be willing to bet you didn't heat it enough, i use a hand held torch with a hot air attachment to heat em slowly and evenly. It sounds like your oil didn't all make it to the bottom and isn't making it into the lower chamber. Before i bought the persei/herc i noticed id have to re-prime my eo carts once a week or so, once id used a bunch of oil the rest would be stuck up higher in the chamber and id have to heat it to re-prime and get it to the bottom again.