Originally posted by Na Horuk It comes down to optical designs. Other than aperture, some of them 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.
No fully true, as short register distance make it possible to design more compact lenses than on a longer register distance.
For mirrorless, especially wide angle lenses can be made considerable smaller and lighter, but on DSLR longer focal length can be made smaller as they sit further away from the sensor. Once the focal length is longer than 100mm, the difference between them are small. Below is Sony A7rIII with Zeiss Loxia 21/2.8 and Nikon D850 with Zeiss Milvus 21/2.8. Both lenses are top performers in it's class.
If lenses for DSLR are more compact then on mirrorless, in general that mean that the DSLR lens is based on a very old optical formula.
Quote: 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