the default boot images are fixed size depending on your AmiKit MorpheuZ settings (HD 1366x768 or FullHD 1920x1080).
If you use different resolution for your AmiKit desktop, the default boot image is always shown in HD or FullHD and then, when AmiKit is ready, the screen mode is switched to your actual AmiKit desktop resolution, which is in your case 1600x900.
To avoid screen mode switching, you can either disable the boot image (MorpheuZ > Startup Control > Startup Picture) or you can create your own 1600x900 boot image. To do so, run AmiKit:Prefs/ShowAmiga96_Prefs program and select "Images". Disable the "Alice" one and press Add button to add your own. Later you can experiment with the progressbar - click the "Effects" tab and edit "Progressbar" entry. You might want to check Alice's progressbar settings for inspiration.
Hope it helps.