First of all, the very formulation of the question suggests that there should be a dating site for plus size people who specifically want to date other plus size people. Facebook features: everybody is welcome to comment on profiles, blogs, and forums. Today people found a way which helped to monetize their work.
Not sure where to start? Those swipe-right, swipe-left sites are just much simpler to set up. Just like normal size women, big women are not always easy to approach.