How to grapple someone when there is a large weight difference?

I'll give the question then give background on how I got here. How can you effectively use grappling techniques from BJJ and other arts when there is a large size difference?

So I practice a lot of different martial arts(M18), and so does a friend of mine(M24). Now, my friend practices BJJ much more consistently than I do, and is more skilled on the ground(I think, I'll explain why I'm unsure about this in a second). Basically we spar weekly and are relatively evenly matched when it comes to striking(On a good day I'm generally faster than he is). We've been sparring over the last 2 years consistently, so we know how each other thinks in matches and can usually anticipate what the other is going to do.
All this brings me to my problem. My friend is like 100lbs heavier than I am. When he decides to get me to the ground, when I am unable to sprawl or get back, we go the ground. On the ground we both do our best to disrupt each other from getting a submission(This includes punching each other as we both wear MMA gloves and both wear shin pads). Now usually against other opponents who are similar sized to me, we can have a more even match even in the ground. With my friend we've both noticed I'm usually unable to do much because of his sheer size advantage on me. Now I know there are weight classes for this kind of thing in sport for exactly this kind of thing, but I'm interested to hear what people think. How can you effectively use grappling techniques from BJJ and other arts when there is a large size difference?