In noticed there was no answer to this question, so let me add my 50 cents.
I used to compete a lot in the open weight classes, me being about 70kg (at that time) and many of my opponents 90kg even up to 130kg.
What I learned from this... try to stay on the top... once you get stuck in a knee on belly it's kind of game over... you strategy should be to outrun your opponent by being more fast.
For example, it should be much easier for you to shoot and go for the back or to make sure you stay away from being locked down in the grips.
If you somehow get stuck on the bottom try maintaining a full open guard, so you have your legs to control the opponent or go all the way deep half guard for example.
Again you do not want to be somewhere inbetween as this is the point the bigger opponent will lock you down.
And in competition is you are way behind, try to give up an arm on purpose so you can anticipate the escape.
I used to have a student that was very good at this, so good we were not going for his arm anymore even when he was giving it.
Anyway, this might bring you back on top if really needed.
Have fun.