AleskbI believe it has less to do with case size & more to do with feature differences. For instance, the original Monsters have screw-down crowns & 200M WR, whereas the Baby Monsters have push-pull crowns & 100M WR. Does that sound about right to the real watch experts on MD?
HenryWaldenAdding to that, the 5 Sports designation is indicative of the lower WR and non-screw-down crowns as mentioned by @Polecat usually along with see-through case-backs.
AleskbAlso belief the naming has to do with the lower WR rating of 100m and being a 5 sport family member. To opt out of that confusing naming for a 43 mm watch I prefer the other name reference; Ice Monster, for the silvery dial and white "teeth" indecis with blue outlining.