At 400mm the size of the lens is 95% defined by the aperture and not the sensor size. A 400mm 5.6 lens needs a 71mm diameter front element, no matter the sensor size. And all other lens element need to be sized accordingly.
Smaller aperture, like 6.3 and less corrections would make a lens smaller but dropping 1 stop would be too much. Don't think anyone wants a 400mm F8 lens.
I realized the aperture would impose that sort of constraint. I was more uncertain about the length. If the length could be reduced significantly, that might be worthwhile, at least to some people (maybe not enough).