[thxbb12]

System equivalence: it's very simple really.

As Klaus mentioned it's about the system (lens + camera).

 

These 3 systems and settings will give you equivalent results (same FOV, same noise, and same DOF):

• Nikon D4, Nikkor 24mm @ f/2.8, ISO 400
  
• Olympus E-M1, Panasonic 12mm @ f/1.4, ISO 100
  
• Fuji X-T1, Fuji 16mm @ f/1.9, ISO 300
  

If manufacturers where to release "slow" prime lenses for FF (say f/2.8), they would probably be smaller than equivalent MFT (f/1.4) or APS-C (f/1.9) lenses. Note that when I say equivalent here, I mean system equivalence.

 

Likewise, a FF system with a 24-70 f/2.8 lens would be equivalent to a 12-35 f/1.4 in MFT land. Such a lens would be huge and very costly. 

The smaller the sensor size, the less it makes sense to produce very fast lenses (from a cost and size perspective).

 

I find APS-C to be a fairly good comprise between DOF capability, size and cost.

MFT is very interesting for very long lenses and macro.