Removing the mirror does allow for a lot shorter registration distances and that has its own set of strengths and weaknesses.
I am not a lens designer, but my understanding is that your smallest lenses will be at focal lengths close to the registration distance. That is why the DA 40 is so small and why the smallest Sony FE lenses should be smallest around 18mm (they don't have any FE primes at this focal length at this point). Lens designers can make choices not to clean up aberrations or, to have slower apertures and these will also tend to have a smaller lens.
With zooms, mirrorless just doesn't offer a whole lot of advantage. The G master 24-70 is 87.6 by 136mm with a weight of 886 grams while the Pentax 24-70 f2.8 is 88.5 by 109.5mm in size with a weight of 787 grams. Certainly at a minimum, when it comes to zooms, you can safely say that if they are of similar quality and cover the same focal lengths, they will be similar size (if not bigger) on mirrorless compared to lenses designed for SLRs.
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