One DS9 episode had a small tug towing an Excelsior class, so they're not so "weak".
That's true, but I got the impression that was a pretty good-sized ship. The Pilotfish is a little ship with small, relatively low-power engines. It normally tows small freighters, scouts, etc. For something as big as a starship, it would take more than one.
That's my story and I'm sticking to it!