I understand that you wanted to compare lenses with a similar focal length range on APS-C, but comparing full frame lenses with an image circle about twice the surface area of APS-C lenses isn't fair for the full frame lenses to start with, at least in terms of size, weight and filter size. You still have a point in the fact that Canon and Nikon don't have a lot of APS-C options in the range of the DA 20-40mm (the 17-55mm F2.8 lens they both have in their line-up is the only one, actually

).
To be fairer, you could also have included other lenses with a similar
equivalent focal length range, but in APS-C (DSLR or mirrorless) and in FourThirds and MicroFourThirds mount.
So I added a few APS-C and 4/3 lenses to the ones in the original post. And for purposes of a "decent" comparison, I didn't include any lens with a max. aperture smaller than F/4, or any lens with a focal length lower than 28mm or greater than 70mm in full frame equivalent, so that it's close to the limits of the Pentax's 30-60mm in FF equivalent focal length range. Here we go:
- Canon:
no lens in the 28-70mm FF equivalent focal length range and/or with a max. aperture of F4 or faster.
- Fujinon:
no lens in the 28-70mm FF equivalent focal length range and/or with a max. aperture of F4 or faster.
- Nikkor:
no lens in the 28-70mm FF equivalent focal length range and/or with a max. aperture of F4 or faster.
- (Olympus) Zuiko Lens 14-35mm F2.0 SWD
Filter: 77mm — Size (D x L): 86.0mm x 123.0mm* — Weight: 900g* — Weather sealed: YES** — Motor: SWD — Stabilizer: NO, in-camera
*: Plus the length and weight increase of a MMF-2 or MMF-3 adapter when used on MicroFourThirds cameras.
**: Only with MMF-3 on MFT cameras (MMF-2 not weather sealed).
- Panasonic:
no lens in the 28-70mm FF equivalent focal length range and/or with a max. aperture of F4 or faster. - Pentax DA Limited 20-40mm f/2.8-4.0
Filter: 55mm — Size (D x L): 71.0mm x 68.5mm — Weight: 283g — Weather sealed: YES — Motor: DC — Stabilizer: NO, in-camera
- Samsung:
no lens in the 28-70mm FF equivalent focal length range and/or with a max. aperture of F4 or faster.
- Sigma ART 18-35mm f1.8 DC HSM
Filter: 72mm — Size (D x L): 78mm x 121.0mm — Weight: 811g — Weather sealed: NO — Motor: HSM — Stabilizer: NO (in-camera on Pentax recent DSLRs)
- Sony:
no lens in the 28-70mm FF equivalent focal length range and/or with a max. aperture of F4 or faster.
- Tamron:
no lens in the 28-70mm FF equivalent focal length range and/or with a max. aperture of F4 or faster.
The Sigma 18-35mm beats the DA 20-40mm on aperture speed and wide FOV, but is much shorter at the tele end and doesn't have weather sealing. And it's a LOT bigger than the Pentax 20-40mm.
The other one, the Olympus Zukio 14-35mm F/2 is as big as the Sigma 18-35mm, has similar aperture and size, and is even a bit heavier. But it does have weather sealing.
Still, the DA 20-40mm offers the best option in the pack when it comes to traveling light or using a non-obstrusive lens on your camera, at least in this very limited focal length range.
But...
If I could've included the Fujinon XF 18-55mm F/2.8-4 R LM, it would have been a close call, with the Fujinon having a better focal length range but no weather sealing... Same for Panasonic's Lumix G X Vario 12-35mm F2.8 Asph. / Power OIS lens, with the Panasonic having a faster aperture, wider FOV but shorter tele reach and no weather sealing either. But both go beyond the 28-70mm FF equivalent focal length range.