I think the sizes are based on the default histogram binning, which is largely based on the number of rows. I'm afraid I don't see a way to override that. Summarizing the data so that there are fewer rows may help. For example,
use Distribution to make histograms of those two columns
adjust the bins
Save Level Midpoints for each histogram
Table > Summary by the two midpoint columns, which produces a new table with those two columns plus an N Rows column.
Graph the two variables as X and Y and put N Rows in the Freq column.
If you find binning that works, you'll probably want to edit the last germline midpoint so it is less than 1 so those values don't spill into the next cell.