My larger tabby was fine with the Genie Dome. I had read that large cats may not fit.
Manny weighs about 15 lbs., and he doesn't have a problem with it. As a matter of fact, his coat's a lot cleaner after the switch from litter. Their old (newer too really) litter box was as good as you could get as far as walking in the top, so my three cats were fine with the dome, including the largest one.