Ok, I'll explain it one more time... LOL
LOW Reps (at a weight you can only do that many of) of around 3-4 reps will work the class 2b and c muscle fibers... these are the larger of the muscle fibers... so naturally, when you hit them with the higher weight, lower reps, you'll make them grow... they are hit fully this way... you will get less effective growth in any other manner.
HIGH reps at a weight that demands it... 8-12 reps or so... will work class 1 and 2a muscle fibers... these are considerably smaller... but do account for some of the muscle size... so if you are always doing the low reps, you might want to consider doing the high rep workouts for a month or so to get some gains you wouldn't get otherwise.
In my experience, many people do around 8-10 reps regardless, and maybe ONE set go for the 4-5 reps. Very few people do a warm up or two and then knock out 3-4 heavy sets.
That being said, if you want to get BIGGER, go heavier... for the LOWER reps... push yourself... and it will come.
C-ditty (Always consult a physician before workouts if you have any preexisting medical condition)