If you have a small ship that has 2 generators, and another larger one that has 5, and they both have the same power supply, the ship with 5 generators will have more powerful shields.
The smaller ship would have more efficient shields (may be faster recharge rate?), but not more powerful ones.
That might not make a difference. A single shield generator covering a twenty meter sphere has less of a load on it than two shield generators covering a hundred meter sphere. And load makes all the difference in deflector shields, since an over
load is the one thing that can cause them to loose strength (or, if the shields don't overload, then your engine systems will start blowing out from the strain).
Besides which, the Miranda class has about the same internal
volume as the Constitution class; if you squashed the Constitution's secondary hull and slapped it on to the back of the saucer module, it would only be slightly bigger than the Miranda's main body. So I doubt she has fewer shield generators... though this is assuming starships really do have large shield generators when they could, in fact, have collimated arrays of small deflector elements built into the hull like 24th century phaser arrays.