I have given this some more thought to this topic & I now realise that I was WRONG. Apologies to all.
Thinking about it: the expanding gas produces a force that spreads in all directions, combustion chamber & piston top. If the piston diam is increased, all else being equal, the downward force will be greater because of the increased area. I do not why I didn't see it earlier...
Newbomb Turk said I was bull headed & he is correct....but I am actually proud of that.