If you have only a few bubbles in your bubble chart and do not specify a Z value, the initial bubble size is monstrous. The bubble size can represent a Z factor, such as Revenue, ROI, Risk, etc. But, you will often need to scale the bubbles so they are more appropriately sized and aren’t too dense.
To scale the bubble size, right-click on a bubble so the entire series of bubbles are selected. Choose Format Data Series. Enter a scaling factor for bubbles.