Head compartments on most recreational boats used to be quite small before "cabin" boats became floating condos...and still are on smaller boats. The discharge hole in a marine toilet bowl is always in the center of the bowl, making the front-back measurement of larger household bowl longer in front and in back...the toilet has to sit further from the wall behind it, which makes space in front of the toilet a lot tighter...sometimes TOO tight to give knees enough room, and sometimes makes it impossible to close a head door that opens inward. So compact marine toilets--which is what almost all production sailboat builders use--always have what the industry calls "marine" bowls. The good news is, if you have the space for a larger household bowl, new toilets now offer a choice of a "marine" or "household" bowl or you can just swap out the "marine" bowl on an existing toilet for one...Doesn't have to be the same make/model toilet...Any bowl that has a 4-bolt mounting pattern will fit any other toilet that has the same mounting pattern.
I originally had the Jabsco manual compact toilet. I compared the dimensions of their compact model to their "household size" model. Every dimension is only slightly larger for the household model. I was able to replace the compact with the household size which is much more comfortable. Most boats could probably make this change.