Originally posted by Class A FF glass is only bigger (and potentially more expensive) when it is faster. FF glass that is equivalent to APS-C glass (i.e., 70-200/4 on FF vs 50-135/2.8 on APS-C) is not bigger and typically cheaper.
This is based on the irrelevant for photography "equivalency" myth.
These lenses are not equivalent for the following reasons.
1) Different formats give different DOF at the same magnification, focus distance and numerical aperture. Trying to equalize this is meaningless as you need to take it into consideration in real life photography.
2) The lenses in question have the same minimum aperture giving different DOF on different formats - hence not equal. That is if you buy the argement that they are equivalent in the first place which they aren't.
3) they give different exposure at the same DOF (see point 1) - hence not equal in real photography where such things matters. Eg. shooting wide open in order to stop motion you ned the same max aperture (but 1.5X focal lenght for FF to get the same magnification) to get the same exposure. The differences is what is inherited in the formats which you can't equalize you away from without making the formats equal where the exercise becomes pointless. .
4) They let in different amount of light into the finder (wide open metering) hence the fastest will help in focusing and composing under difficult conditions - hence not equal.
5) "Equal" lense after the Equivalency" formula would typically make different magnification when close focusing - hence not equal in real life photography. An illustrative example is macro lenses FF that goes to 1:1 will give different results dependent on the format; 50% larger than 1:1 on APS even if the aperture and focal lenght are "equalized" to match a longer macro lens.
It is obviously the case that larger format needs larger lenses. If this was not the truth then the lenses for the Pentax Q would have been the same size as the lenses for the Pentax 6X7. This happens in a linear manner depending on format size.