Smaller? The smallest ST hardcover I have (other than a few MMPBs that I had custom bound by a local library-specialty bookbinder) is the Gregg edition of The Klingon Gambit, and it's still slightly larger in page size than the MMPB original (albeit maybe slightly thinner; if the pont-size of the type was held constant and the column size were expanded to fill the page size, that would account for a lower page-count).
So far as I'm aware, unless the fact that an edition was abridged is obvious (e.g., the Little Golden Books picture-book of Baum's The Road to Oz, or the Random House pop-up edition of Baum's The Wizard of Oz), I would expect a publisher to state, right on the cover, that an edition is abridged; otherwise, they might get sued for misrepresentation.
Incidentally, one of my custom-hardbound MMPBs is Spock: Messiah, by Cogswell & Spano. Not a terribly good book, but (other than the children's novel, Mission to Horatius), it was the only the second adult-audience ST novel published (after Spock Must Die), and the first single-author volume of adult-audience TrekLit not by James Blish. So it is of some historical interest.