Most of the time bigger guys dont have a good high guard. The most dangerous closed guard ive seen from a strong guy involves the anaconda choke from guard, that can get me.
Against this case study, id say relax and dont try so hard to break - he will most likely have to open to attack. Let me know if that is not the case and your dealing with a high level closed guard, but let him open and force half guard. One of my go-to's, cause not a whole lot of people at my school can fuck with me without opening their guard.
Now against big strong guys in general, when NOT in their guard, OMOPLATAAAAA