I un-ticked 'Advanced' and saw your expression screen.
The problem is, the ;left chart expression Sum([Charge]) and the right chart expression Sum([Actual Charge]) are relative to themselves so the size of the bubble does not change. What I need is for the right hand chart expression to be 3.75x smaller than the left hand chart as the Sum([Charge]) = $111,000 and the Sum([Actual Charge]) = $29,751
I'm afraid there is no way to it. Each chart has own relative scale for bubbles.
The only workaround I can see here - to add technical falue (via script) to each value of both your dimensions, lets say max number, it will be on both charts. Then make this bubble transparent using conditional colour. Still not a good way.