Upgraded to IS2021. Pure Installscript project. Now Altimage.bmp is smaller

[markhug]

Pure installscript project.
Using a tiny "test" project in order to illustrate the problem (problem occurs in all my 2021 projects).
Using Slate skin
Opened an IS2015 project in IS2021.
Built a release
Ran the build (setup.exe)
Expected behavior:  The altimage.bmp to display filling the left side of the dialogs like it does in previous IS versions.
Actual behavior: The altimage.bmp file displays smaller.

See images below which show the problem.

Any ideas?