When I did BJJ, I was one of the smaller guys - 5'6", 150 lbs. I had just gotten done with running track, so I used speed and stamina to my advantage, and could wear down an opponent by struggling constantly. What I eventually realized was that I was using those to succeed rather than technique, and forced myself to slow down and think about technique. When I did, I noticed that my opponents would use alot less strength, since they didn't feel like they were in a fight the whole time either. So my advice is this: if your opponent is using physical attributes to win, then use yours. If not, then use technique.