Played around with the splash screen.
It is set as a parameter to the java JRE but can be accessed within the java-code.
They already do this in Arduino\app\src\cc\arduino\view\SplashScreenHelper.java in order to draw the status text (Loading this...doing that...Starting ...) to the bottom of that splash screen.
I was able to scale the image (but it was clipped to the size of the old splash screen. I think its a matter of bounds I have not updated).
How ever - I noticed that it took a while until Java resized the Image. Felt like some few seconds (3 or 4) on my very fast PC. So I think this would not make any sense if the splash screen is resized seconds after the original one is displayed.
Maybe something comes into my mind later.