mY BEST SMOOTHERis my Mujingfang polishing plane which of all my planes is the lightest. The plane is amazing, better than any of my smoothers -#4 w Hock blade, # 4
1/2 with L-N blade, Shephard Tool Spiers 7, St James Bay Norris-. That I think puts the weight issue as Jeff G. said to rest.
Remember that the stanley bench planes are cast while the original (Spiers, Norris) infills are dovetailed. Having built a dovetailed smoother and a cast infill smoother, I believe that the infill was added not for mass but as a structural component. Both the front and rear infills support the sides to prevent outward and inward movement. Think of build a dovetailed drawer with no bottom or back, just front and sides.
I feel that the infill bailey on the link you provide benefits from the Sorby blades mass (less chatter, etc) AND the fact that the thicker blade provides for a tighter mouth.
regardless of bed type - adjustable frog or infill fixed- the last portion of the blade in a bevel down plane is unsupported. So a thicker blade, as the infillshave would, in addition to the thicker chipbreaker, provide a more stable (slightly) cutter.
In general the brittish infill smoothers may plane better, but as Jeff G said it may just be folklore that exaggerates the difference.
Scot