The "line" is simple. It just comes down to why. If someone loves a huge belly because it's pregnant, then they have a pregnancy fetish.
I love huge bellies in general, but a belly filled with air is way less exciting than a belly full of babies. Even at hyper sizes, I'm more drawn to a belly thats pregnant than anything else. Helps that I do love the more "intense" (according to many preg artists at least) parts of pregnancy too, like stretchmarks and birth.
Meanwhile, there's people who don't care why a belly is big. Or the types who try to twist pregnancy into "unique" (boring) ideas like "she's pregnant not with babies, but with happiness!!!"