The size of pie charts differs based on the amount of labels attached to it (outside labels). This can be observed in this fiddle and the following image:
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Of course, the available size for the chart differs based on how many labels are attached to the pie chart. However, this could be solved by introducing a fixed padding around the pie (the circle) which is reserved for labels even if the chart only has one label.
Adding such a padding will result in generally smaller pies (again, I’m referring to the circle) and seems to be a waste of space if you have only a single pie chart on a page. However, we have multiple pie charts on a single page and it would be great if they had a consistent size.