less has a flag for that: (the gnu version does at least)

       -F or --quit-if-one-screen
       Causes  less  to automatically exit if the entire file can be displayed on the
       first screen.

So one could just pipe everything through `less -F`. If a different $PAGER is preferred, well that'll be a little more involved.

(Also handy: add in `-X` to leave the contents on the screen when you exit less)