The HiDPI higher resolution UI comes into effect when your Windows Display scaling is 175% or more. Below that value the larger UI elements are too invasive.
If you need the larger UI elements you can do two things. The first, as you've seen, is turn off the HiDPI support in the ArtRage Preferences.
The second is to exit ArtRage, then right-click on your Windows Desktop and increase the Display Scale value to 175% or higher.