I was able to reproduce your result using:
under
Try some other arrangements to see if it resolves your concern.
I think I managed to figure out that it has to do with the default setting of automatically changing font size with the size of the legend box. As I expanded the legend box the bubbles in the legend seemed to align better. So I guess the answer lies with the "fitting strategy" while working on the legend (I set it to "Adjust columns" for now).