It comes down to optical designs. Other than aperture, some of the elements are:
a) Lens complexity. For example, a superzoom lens might need more elements. Or ones with internal focus or Tilt/shift tech. There are also different base designs that come from different optic lineages. Different lineage are like Tessar, Planar and other designs. They define the lens size and affect the output. You can have two 50mm primes, and they have the same FoV, but their actual photos will look different (bokeh, colours, contrast, etc.)
b) Lens quality. A lens with many corrective elements will be bigger. This is why a lens with extreme sharpness, corner to corner sharpness, low CA, will have more elements. Some claim that having many elements is actually bad, but that comes down to preference.
c) Format. FF lenses
in theory would be bigger than APSC lenses. And both are bigger than the smaller Q format. 645 medium format lenses are bigger. Part of this is the lens mount itself - Pentax Kmount allows some interesting pancake designs like DA 40mm, DA 21mm. But it makes the camera "bigger" (require space in the mirror box). Some mirrorless camera mounts have much smaller mirror box requirements (Register distance), but this means their lenses are generally larger. The difference between FF and APSC is almost not noticeable, especially since many APSC lens designs are derived from FF ones. Difference between Q and 645 is more obvious.
What do users want? Top quality in a compact package at a low price. Of course, having all 3 is impossible and manufacturers have to choose between them. Lately lens size is increasing. Many manufacturers seem to be ignoring lens size and price just to produce stunningly sharp lenses, which has its own pros and cons
Last edited by Na Horuk; 01-15-2018 at 07:58 AM.