High memory usage for multiwindow apps

I found that with default Windows renderer even simple app (but with many windows like custom dialogs, menus etc.) eats too much RAM, up to 500 Mb after start.

It depends from installed memory size, but not linearly.

Few tests for my app:

Virtual machine with 512 Mb, it eats ~80 Mb
Virtual machine with 1 Gb, it eats ~110 Mb
Virtual machine with 3 Gb, it eats ~110 Mb
Machine with 4 Gb, it eats ~160 Mb
Machine with 4 Gb, it eats ~500 Mb
Machine with 8 Gb, it eats ~250 Mb

If I choose another render (Qt::AA_UseOpenGLES), it uses ~80 Mb
On macOS same app uses ~58 Mb

Unfortunately, I can't use OpenGL ES because of some rendering issues, like QTBUG-54451

Minimal example attached, on my machine it uses 33/103 Mb for 11 empty windows.