For example the Nikon Z 50mm f1.2 is 1090 grams, 150mm long, and has a 82mm filter size. The Canon RF 50mm f1.2 is 108mm long, but the other dimensions are similar.
Compare that to a Leica Noctilux 50mm f1.2 with a Techart, Megadap or similar adapter (available for Z and E mounts) for autofocus abilities: 405g lens +150g adapter = 655 grams, 52mm lens + ~11mm adapter = 63mm long and 49mm filter size. A little more than half the numbers in all dimensions.
This link approximately shows the size differece (the M to L mount is indeed smaller than the M to Z or M to E autofocus adapters, but the difference is small)
All of these have the same focal length (50mm), max aperture (1.2), and autofocus. So why do these newer mirrorless lens designs have to be so much bigger and heavier than using an old manual lens with an autofocus adapter? Sure the autofocus speed may not be as fast with an adapter but why can’t they design a native autofocus large aperture lens that is tiny like the Leica M lenses. Clearly it is possible to do so.
Simple (not really): Every time a photon passes through a medium, like say - glass - that photon gets distorted, at least to some extent. You must now correct for that distortion and you do so by… wait for it… passing it through another medium.
Uh oh. You’ve distorted it in an entirely new and exciting way. Which must now be corrected for. Guess how that’s done?
It’s enough to make an engineer weep.
Read this: https://www.dpreview.com/opinion/9236543269/why-are-modern-50mm-lenses-so-damned-complicated
So photons just started doing this in recent years?
They have always done that.
The less complicated the lens, the less it controls for flaring, aberrations, loss-of-sharpness etc etc.