1. The high ram usage isn't related directly to the ImageViews.
2. Unless the process runs out of available memory then you should ignore it. The garbage collector will allocate RAM when available.
If it does crash then you should debug your program and see where it leaks memory. Tip: only do it in release mode.
what's strange is that I didn't do anything different in both :
I created an ImageView / Label in DesignerEditor -> Imported the image in the Files tab -> in Background Drawable I set BitmapDrawable an image and run it in Release
One classic cause of this apparent excess memory use is that images comprise a small managed (Java) object and a larger unmanaged native image. The Java garbage collector doesn't know about the large native object and so doesn't see the need to run a garbage collection when a reference goes out of scope so the memory use builds up until a collection is made. Don't obsess over this, it will get collected at some point.
If you do actually end up with out of memory problems by using lots of large images then you can call a garbage collection yourself but this is not really advised except in extremis. Garbage collection | B4X Programming Forum
We run several servers at work which run Java (via TomCat). Java is known to allocate huge amounts of RAM. Never had problems (although it's "scary" at the first look).