You're problem is that a bubble chart has a maximum allowable bubble size. The largest plotted bubble value is scaled to this size. Then every other point in the data is set relative to that largest value.
I'm having a similar problem in trying to get a bubble with value = x to be the same size across multiple sample sets... VERY frustrating!
My suggestion is to set the maximum allowable bubble size as large as you can. Then add one hidden data bubble that is much bigger than your data will ever reach and see how it looks. Adjust this large data point until your data bubbles fall in a range that's satisfactory to your data.
