Spent a while looking into this and couldn't find a clear answer, so here I am. Specific and thoughtful opening messages get responses at a far higher rate than generic openers.
BBW-positive platforms have grown significantly and their communities are generally very welcoming. OkCupid's free tier is still surprisingly usable compared to what most other apps offer for nothing.
Profile quality matters more than which platform you choose — a well-written bio beats a hundred passive swipes.