Do you have some examples of lenses with comparable optical properties that are smaller in mirrorless mounts?

AFAIK demand for corner-to-corner sharpness wide open has made lenses huge NOT whether there is a mirror or not. I have a bunch of old school Nikon primes that are tiny compared to modern lenses but are essentially "portrait only" lenses until f/4.

I think the Zeiss Loxia (mirrorless) and Milvus (SLR) lines are an interesting comparison point, since they're both from the same manufacturer, both manual focus, and should both be comparable quality.

The Loxia 21mm/f2.8 is 72 mm long, and 394 g. The Milvus 21mm/f2.8 is 95 mm long, and 735 g (Nikon mount)

Once you go to a higher focal length, keeping the lens compact requires sacrificing something, i.e. slower glass:

The Loxia 35mm/f2 is 59 mm, 340 g. The Milvus 35mm/f1.4 is 125 mm, 1131 g.

The Loxia 85mm/f2.4 is 95 mm, 594 g. The Milvus 85mm/f1.4 is 113 mm, 1210 g.

So, I'd sum it up as, at wider angles, the lens designer probably has a bit more flexibility because of the absence of the mirror box, but at longer focal lengths it matters much less. It's also interesting to me that Zeiss is focusing more on slower lenses that retain the compact form factor. They do have a 35mm 1.4 FE mount, which is huge, and Sigma recently released their FE versions of the Art prime lenses (which are just as big as their SLR equivalents), but the bulk of the Batis and Loxia lineups are f2 and up.

Seems to indicate that while there was indeed a lot of demand for huge, extremely sharp f1.2 or f1.4 lenses in the SLR world, mirrorless customers generally seem OK with trading some of that speed for less weight.