drop bear
Sr. Grandmaster
It definitely doesn't make a difference for most Submissions/finishers. But when it comes to positional availability, I struggle against people with less BJJ experience than I. It's a lot tougher for me to hold mount or side control, and I've learned to try to get things from guard and/or while they transition between situations, as I have a much tougher time holding positions than those larger/stronger than me.
Admittedly, I'm also much less experienced so that might be part of it, but given it'll take a year or two of dedicated jujitsu training to pass my experience (I've been doing it on/off with limited dedication for a few years), it's something to keep in mind when discussing with people that are smaller than average.
Someone I train with learned to focus on leg locks. It turns out most people don't focus on them, so she's able to get leg locks regardless of her opponent's height/weight (at least up to a certain experience level).
You don't hold people down. You move so that you are always on top of them.
And for mount. Just starfish.