Someone correct me if I'm off base, but I think it's mostly about total volume and caliber. The longer and fatter, the better suppression. It's a trade-off between looks and function. Generally, the bigger the caliber the bigger the moderator needs to be. All things equal, a .22 isn't going to need as big a moderator as a .30. Also, some guns have better moderation built into the barrel/shroud design than others which will affect the size of moderator needed.