Don't judge a workout by the "pump" or the "feel". It is not a good indicator of the effectiveness.

I would say if you stay below 12 reps or so you would be in size gaining territory. Getting down closer to 5 reps or less is more like strength training. There is no definately boundary so dont' take those numbers as a solid rule. Its more like:

Higher reps --> Size (to a point)
Lower reps --> Strength

Hope this helps,

IW