Sizing Circles for Data Visualization
To correctly size circles (or bubbles) in a data visualization, the designer has to adjust the proportional AREA of each circle. It is a common mistake to adjust the diameter for each circle, because width and height are the common input fields in design software. Charting software, like Excel or Tableau, does this automatically in Bubble Charts, but if you want control over the layout of the circles, you'll need to design your own.
You can download a PDF of the reference sheet here: Sizing-Circles-for-Data-Visualization-Infonewt.pdf