Model Ms have a series of membranes and the internal construction is completely different.
When you take them apart, you see that the Model F is a throwback to an era where each piece was sturdy and replaceable, and that the whole was intended to be serviceable throughout.
The Model M was intended to reduce costs drastically and make for swifter assembly-line production. A bolt-mod is merely a partial attempt to re-create the way it should have been done in the first place.
I am not sure that it is the underlying switch that makes the difference (although the Model F hammers are triple the size of Model M hammers) but rather the entire construction of the beast as a whole.
If Unicomp had the Model F patents as well as the Model M patents, I suspect that the modern Model F would cost at least double what the M costs. After all, if I remember correctly, the Model F sold for about $350 and the Model M for about $250, and those were 1985 dollars.