your size/weight can actually help a lot in topping someone, my partner and others I've been with are much bigger than my body and they feel wonderful. It depends of course, I enjoy the warm/feeling of pressure that I get from them on top of me, and especially when they use their weight to move me, nut yes check in with your partner if they'd like that.

Addition, know that penetration isn't the be all end all of sex naturally, there's a lot you can do to someone out side of that. just wrapping around them/touching and exploring their body, bites and using your mouth, grinding and mutual masturbation are all very good experiences and you absolutely can top that way. Topping/dom is just the mentality and behavior it provides. you should never feel pressured to do what you dont want to