I wondered about that too, my big crabs had to be manually removed because they just kept growing bigger and bigger but my cool porcelains disappeared.
Also some of the very tiny hairy crabs disappeared. And once I put some of the captured larger crabs into the 20g with one of my mantises, I actually witnessed the mantis crackin' away at the carapace of the bigger crabs. It just scared the big crabs, and they ran away, but it didn't split 'em open.
It was actually a weird sound, more like a "ting" than a snap. Like the crab carapace was tinging like a drinking glass.
So perhaps the little mantises are only really able to knock out & eat the littler crabs.