Consider the Grouped Points with Proportional Symbols Map example: here circles are plotted so that their size (specifically, their area) is proportional to the data: in this case, the number of airports.
The legend shows how the size of the circle corresponds to the count of airports.
Is it possible to increase the absolute (not relative to other data; so Size/Scale as explained in this answer is not relevant) area of the circle that corresponds to the maximum value? Suppose that the radius of the circle corresponding to the maximum value is 1.0 em. Can I make it 2 em large?
You can do something like alt.Size('column').scale(range=(0, 1000)) as in this example https://altair-viz.github.io/gallery/airport_connections.html