I'll bet your highest stat determines which kind of snowmen you help out with. I consistently help with the monstrous ones, and Strength is my highest stat. I'd imagine the tiny ones correlate with Reflexes. Has anybody with Perception or Will as the highest stat tried helping out? I wonder if there are 3 other special snowman encounters we can unlock in addition to the Swarm of Tiny Snowmen.
EDIT: Maybe I spoke too soon. This may be random. Got the monstrous one even with base and buffed Reflexes higher than Strength.