Honestly, I have to guess that a lot of size vore content is just… vore content, taking the most logical approach to eating a person: making them bite sized.
On vore? Because it’s very… primal. It’s predatory behavior, tying in well with how size fetish in general, and it’s a way to dominate someone and dehumanize them in a fundamental level, reducing them into food. When you eat someone, you take them, in their entirety, and you never will be separate again. Also, presumably, they’d taste good, and all that fun stuff involved in actual eating.
Mouth play has a lot of similar appeals, but non-lethal, obviously. It also allows you to combine pleasure and fear.
Insertion isn’t really my thing, but it’s probably similar to normal vore, but just… more of a focus on dicks and the sensation than eating.