I believe time and time again that the guy with the better technique will always defeat the strong and bigger guy unless the weight difference is more than 15 pounds. 15 pounds does not make a big difference and even at 30 pounds, you can still defeat your opponents if your skills are substantially higher.
Focus on all the positions and my personal view is to grow from submission defense and into to getting skilled at winning a good position. Only once you can escape every submission of your opponent should you focus on winning good positions such as the side mount and the mount.
Once you can do this easily, then I believe you should focus on submitting your opponent yourself. This is the way Saulo suggests to progress. You can't just attack all the time but you should also be focused on having a good submission defense.
I think it is better to lose some of the positions but escape every submission then winning a bunch of positions and then get submitted several times in a row.
It takes many years to master bjj and you should not think about the time but the development. Focus on who you are fighting in training and practice with different people at your academy, people of different skill level, size and also style.
At my academy I think there are two major styles at work. Those that started from BJJ or from Judo, focus on playing the guard, on sweeps and passing the guard and on the mount.
Others however are very focused on leg locks and winning by submission without valuing the positions they achieve.
Both of these styles I believe I have to get used to and learn how to fight. I have to be able to escape any leg lock attempt they make and then pass their guard. Before trying any submission.
I try to be as relaxed as possible during sparring in my Academy. It does not make sense using strength as many say but I also try to be calm. I think about what submission my opponent is trying to do. As soon as he tries to secure the guillotine choke, I already have my chin down and my shoulder up to make sure he does not get it.
It does not make sense to use so much energy if you can wait, assess the situation, look for an opening in the opponents game and then take advantage of it at the precise moment when he makes the mistake.
I believe this is the essence of jiu jitsu. The youtube match between Bruno Bastos and Lagarto Rodriguez I think best illustrates my point. Rodriguez is calm, looking for the mistake and not spending energy trying to do some fancy techniques.