Rotating iPad to portrait mode does not fill external display

Please can you advise how to get an iPad when rotated in portrait mode to fully fill the display? As can be seen below, in landscape mode, the iPad mostly fills the screen but in portrait mode, it shrinks and does not fill the full display. Please note the following two points:

1. This is not a Dell issue but rather an iOS software issue as it is possible to display both MacOS computers and Windows computers correctly in portrait mode filling the display. The 3rd image is proof of the system working correctly on a Mac.
2. The rotation lock function does not fix the issue, attempting this makes text appear sideways on to monitor or in 2 page up view which is not wanted.

Grateful for your help in providing an alternative solution that can be done from my iPad Pro. Sorry for the incorrect rotation of the pics, the floor/base is on the right hand side of each picture.