By default, the views are tied to each other by the scale/size ratio. To change that:
1. Click on a viewport and head over to the left-hand properties pane.
2. Go to the 'orientation' option, set alignment to 'Independent' and type to 'General' or other perspective.
3. Change the 'Scale' numerical ratio of the view to your choice. Press the enter key to see the changes take effect.
You should now be able to change the size of one view without affecting the others.