The 15s don't have as much space for some reason. I've not seen a smaller version of a 15 except for a Japanese 15 clone in 3/4 size or a Cinderella which isn't really a Japanese 15 clone - only sort of. The 66 and the 201 are a bit bigger under the harp. The 99 and the 185 are 3/4 size and not as big.