Distance can be your friend. Do not let the bigger guy attach, keep arcing to get outside his squared hips (i.e. he is bladed to you while you are squared to him), fast power shots that do some damage.
All those are good plans, but are probably the hardest part of a fight to pull off. Even if you have ample open room with a flat and even surface, it is hard to keep someone away for long. The other advantage the bigger guy has, and one that the "combatives/grab their jewels" types fail to mention, is generally he also has a longer reach making it easier for him to grab on.
And few people have a real understanding of distance and the ability to maintain it. In my Close Contact Handgun course, my co-instructor and I spend about 8 hours teaching and drilling that very skill set, and then we spend the last 4-5 hours of the class with the students under true pressure and that skill falls apart instantly. The most common comments in the debrief are "I can't believe he got that close that fast" or "I didn't realize he was that close". Instructors who don't respect the amount of work it takes to impart that skill are deceiving their students.
Another issue is that the bigger guy can usually take more damage and/or the smaller person has a tougher time developing enough power to hurt the bigger dood. That is something else that needs to be taken into account, and is also one of the reasons that some of us who teach this stuff have been slightly de-emphasizing striking skills in our course work, usually so we can spend more time on things like movement, footwork, and distance control.
One advantage the smaller guy has that none of the "street fighting" teachers ever teach (because they don't have a clue that it exists) is that when it does go to an entanglement, the smaller guy's belt line is lower than the bigger guy, and so he does have a leverage advantage, IF HE KNOWS WHAT TO DO THERE.
To sum up, this is a sucky situation, and it requires a boatload of sweat equity to even come close to developing a real functional response.