OK ... not really sure what you're referring to here Lance but lets put some figures into perspective here:
Sigma 30/1.4(APS-C only): Dimensions (Diameter x Length) φ74.2mm x 63.3mm
Nikon 35/1.4(Fullframe): Diameter x length Approximately 83 x 89.5 mm
APS-C a lot smaller(mainly in percentage terms)
Nikon 16-35/4VR: Diameter x length Approximately 82.5 x 125 mm
Tamron 17-50/2.8VC: LENGTH 94.5mm (no diameter given but filter diameter is 72mm, and the lens diameter looks to be very close to this .. maybe 2mm more)
Note that the Nikon lens is a much less extreme optic in that it's an f/4 .. one stop slower than the Tammy. Sigma's equivalent is about the same size as the Tammy.
At the ultra wide angle end of the scale, we get:
Tokina's 11-16/2.8: Dimensions 84x89mm
Nikon's 14-24/2.8: Dimensions 98x132mm
Pretty much as far as I can see .. the vast majority of APS-C only lenses are in fact on average about 15% smaller in physical dimensions(except weight) to their full frame counterparts.
Apart from the Sigma 50-150mm f/2.8, I doubt that we will ever see any lenses above about 50mm for APS-C sensored cameras(Nikon also has it's 85mm f/3.5 micro), as it appears to be a counterproductive exercise by the manufacturer.
Due to the extreme nature of the designs of wider angled lenses on an SLR format camera it makes sense for manufacturers to produce APS-C only(or 4/3's or mirrorless) lenses .. but once the lens design exceeds the 50mm focal length it seems to require much less in the way of design compromises .. so why limit the longer focal length lens to APS-C only when a larger market bekons!
So one of the not so commonly mentioned benefits of this lens(provided that it does perform as well as the specs say it will), is that a faster aperture zoom lens is possible at the wider end of the focal range and in a size that is relatively small!
Relatively small in comparison to similar focal length lenses with slower aperture values(i.e Nikon 16-35/4 17-35/2.8 , etc .. Canon 16-35/2.8 17-40/4 .. etc, etc) and this lens is an f/1.8!
I'm not sure where you're seeing this point that you've made that APS-C wide angle lenses are not smaller than full frame capable lenses of similar specs?