As you know, when the catalyst kicks in, it gives off heat. That heat causes next-door molecules to kick-off. Pretty soon, the whole thing is cross-linking, going through the reaction to turn solid. Smaller masses (globs of mix) will give off less heat, so the reaction will be a little slower. On the down side, you have to be more precise on your mix ratio.
I'm a poster child for Body Filler Amateur. I know the mechanics of it, but I haven't mastered the knack. Until I do, I'll continue to work with tablespoon-sized mixes on my bumper project. I find when I go golf-ball sized, I end up throwing out about half of it.
Bill
