Why do you need to have a heavy m on a pendulum if the m doesn't effect the period?

m only matters if your pendulum bob is a feather or a small tack or some other object that will be hindered from moving by the surrounding air. Do your experiments in a hard vacuum and it won't matter. The m must have enough momentum or interia to plow through the air and cycle for a while. Period is not affected as long as the pendulum can move.