Numbers Bubble Chart Bubble Radius
@user2421192's comment had the solution: apparently the bubbles are automatically scaled proportionally to the range of the radius data. So by adding a new row at the top of my data, without x or y data, but just a r radius value, rescaled the rest of the data.

Since the radii I have don't vary by much, I decided to modify them with a formula, taking the exp of the original radius r divided by ten (exp(r/10)), and then setting my "scale" radius to 50 * max(radius) (of the rest of the column), which gives some variation in the bubbles that makes it a little clearer whether they're bigger or smaller than the rest.