Apparently. the seat and lid are heavier on the zenith and a little
wider, but will it accomadate a wider butt?

If it's a double-wide, prob'ly not.

Re size, not a Lavac, but on a PHII I think the size dif is only about
an inch or so. Guess that makes a dif for some boats? So when I buy mine
I'll get the "big" bowl.