Viewfinder size is not completely tied to size of sensor/ mirror, and view-ability isn't tied to just eye-point.
The Nikon D1 and D2 series cameras are APS-C, and the VFs are comparatively large, bright and easier to view - as with all of their flagship digital bodies. One item assisting with ease of viewing is a larger exit window - allows easier viewing of the VF image even if the eye-point isn't massive. I can see through the VF easier on my D2x (APS-C) than my D700 (FF) - a combination of better relief (19.9 vs 18mm) and a larger exit window on the D2x. On the other hand, I can see through my D700 much better than small form-factor APS-C bodies with better relief (D700 with 18mm vs D200 with 19.5mm or the K-5 with 21.7mm) due to the larger exit window on the D700.
IMHO the camera manufacturers choose not to include a larger, brighter, easier to view OVF due to price, the desire to keep bodies as compact as possible, and the perception that the consumer isn't concerned with having the larger VF image. Everyone is different when it comes to what they find useable in a VF, and for many people existing VFs do just fine. There are those that have issues with VFs due to the wear of glasses. And there are others, like myself, who have issues with eyestrain trying to look through a tiny dark VF, and end up getting migraine-level headaches when using those camera bodies for extended periods of time. For me, at this time the D1 - D4, D700, and D800 are the only APS-C and FF cameras I have been able to use with minimal pain. Pentax 645 series bodies are easy for me to use as well (but that's a whole 'nother level of cost when talking digital).
Pentax could have included an easier to view OVF on the K-3. I really wish Pentax would have included one on the K-3. Very aggravating, as I prefer the body ergonomics and size over Nikon.
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