When I was first starting out in jiujitsu I was 11 the only kid in class. Everyone else was 3x my size! As I started getting older and better I was able to start getting to top positions a lot more.
But the problem I ran into was if I tried to hold the mount I would get launched across the room. I tried going to fat boy mount and it just wouldn’t work on someone to big and strong.
So I learned that I don’t always have to keep the mount. Sometimes it is better to just switch off to knee on belly than go back if possible or circle to the back. I mix this in with the fat boy mount and I can usually keep to position much easier now.