See, I would think there would only be a few more Mirandas than Excelsiors. And somehow, I'd imagined even fewer Oberths than either of them - a few built in large isolated groups.
It seems to me that the larger the ship, the fewer are built. I can imagine thousands of the smaller Mirandas and Oberths being built, as they do a lot of the basic scouting and scientific missions that form the bulk of Starfleet missions. I imagine that in a lot of stories, once the Enterprise does it's thing and moves on, the smaller ships come in and clean up after them.
You probably aren't going to have the Enterprise-E mapping an L-class planet in the back-end of nowhere, or cataloguing one of the thousands of nubulae or pulsars that must be knocking about in space - that's what everyone else does whilst the Enterprise is off having adventures on television. I suppose most of what Starfleet does is actually pretty boring, but we are lucky to see the biggest and best ships with the most exciting missions.