A larger diameter hollow component is definitely stiffer in bending resistance than a solid smaller diameter component of the same weight. This explains the reason for hollow larger diameter axles which have been used for years. But hollow motor mount bolts are more of a puzzle, as they appear to be in shear where strength is related to area and tensile strength. It might have something to due with the larger bearing area of the larger diameter bolt head and nut against the weaker parent frame material, especially aluminum frames which are more susceptible to fatigue failure.
But Canadians (thanks for the reminder, Stan) are thinking about making all our steel components hollow to make equivalent items 25% lighter - - take that Trump! (And we'll probably invite all the DACA immigrants up here, as well.)
Tom