The Magnify command can not change the size of the points. Instead you can repeat the command with the same data but a larger point size. Then you can specify where the data is shown.
For example, here are two points commands with the same input data. One is set to 3 times the point size. They both have been edited in terms of when they are viewed in relation to the magnify command.
The first is only visible in the main graph. The second is only visible in the magnify command.
Notice on the left near the custom icon, there is an indicator of what was chosen. A label was also added.
Here is the resulting graph.
Does this help?