I'm not sure that "proc gplot" bubble charts have a way to guarantee that the bubbles are scaled the same in different charts.
When I need to have total control over bubble sizes (whether in gplot or gmap), I use annotate's "pie" function. An annotated pie, with style='solid' is a solid bubble, and style='pempty' is an empty circle. I "normalize" my data values against a maximum, and then use those values to control the 'size' (radius) of the pies.