Why are SLR lenses so much larger than Sony E-mount lenses?

Question

Most lenses for Nikon and Canon APS-C SLRs seem to be much larger than Sony E-mount lenses, despite the sensor size being the same (APS-C). This holds for both primes and zooms, and it holds for the diameter, length and weight.

I understand why the NEX cameras are much smaller than corresponding SLRs. But why are the lenses smaller?

I understand that some SLR lenses are full-frame lenses mounted on an APS-C body, but that explains only some cases (please correct me if I'm wrong). What about the others?

Is it because the Nikon mount is designed for full-frame and imposes an overhead for APS-C lenses? This explanation isn't satisfying, either -- wouldn't that overhead be only in terms of the diameter and not the length? Even the diameter needs to be more only at the point where the lens connects to the camera and not throughout the length of the lens. But SLR lenses are both longer and have a greater diameter than NEX lenses. And in any case, this theory doesn't address Canon lenses. Finally, Sony also shares the E-mount between APS-C and FF, and that doesn't make E-mount lenses bigger, so why would it make Nikon lenses bigger?

Is it just the target market -- people who care about size and weight are more likely to use mirrorless cameras than people who don't care as much, so Canon and Nikon don't focus so much on reducing the size and weight, and even if they did focus on it, their users wouldn't care as much as Sony users?

Is it something special about Sony, or about the E-mount? Are Sony A-mount lenses as bulky and heavy as SLR lenses, or as small and light as E-mount leses?

Or is it something about mirrorless cameras? I fail to see how the camera technology affects the lens, but I wanted to mention it as a possibility.

So, what is it? Why are lenses for Canon and Nikon APS-C SLRs so much bulkier than NEX lenses?

Answer 1

Actually, they aren't much smaller. Lens size for similar formats is actually pretty comparable, if you look at similarly speced APS-C vs. APS-C designed lenses. Take the examples (on camerasize.com) of 18-55 kit lenses, the 55-210 vs. a 55-250, or crop 35/1.8 lenses.

The size difference is mostly between full frame and crop lenses. But if you look at a full-frame e-mount lens vs. a full-frame EOS lens, say, a 24-70/4, again, the sizes are pretty similar.

The mirrorless lenses tend to get smaller when the sensor (and image circle required of the lens) gets smaller. As in micro four-thirds vs. APS-C.