It is obvious that, in English, the phrasal verb "get up" (meaning to awake and move out of bed) and the idiom "raining cats and dogs" (meaning rain strongly) have different depths of idiomaticity. By that, I mean that the meaning of "get up" bears a different relationship to the normal meaning of its constituent parts than the idiom "rain cats and dogs". Why is that (something historical or intrinsic to the phrase?), how did it arise, etc?
My question is what theories of idioms address this kind of difference among idioms. Thanks.