Are there certain unspoken rules for intimacy when it comes to a couple with different body types?

So I'm pretty curvy and thick (especially on the bottom) and the guy I'm getting closer with is taller but also way thinner than me. And so I started thinking, if we got intimate should I always take the bottom? Because I'd probably feel like a whale if I straddled him or tried to take the top. Would he think the same? It's kind of embarrassing... 😞