I consider this a bug with how HighCharts sizes its bubbles, which appears to be non-linear...
If you have two points on a bubble chart, with sizes 5 and 10, and set the minSize and maxSize for bubbles to be 5 and 10 it looks good. The 10 is clearly double the size of the 5. This is good.
If you add a third point with size 20 and change the chart's maxSize to 20, the middle point, 10, shrinks and no longer has a valid reference size to the other two points, i.e. it is not exactly half the size of 20 nor double the size of the 5. This is bad.
This is true if sizing with percentages or your own math and doing pixels.
I have exactly recreated this here:
https://jsfiddle.net/arckex/3kwnu69r/