Tweak image resizing process
You have the scripts.min.js file that is used by the theme, but there's also a development version which you would need to work with. If you go through the file you'll see the functionresizeProjectCentered(). This function is the one which resizes the "fit" projects. You will notice that it calculates the height based on the hResize value which is currently at 40. Decrease that for starts. The height has also a 100px manual margin. Try decreasing that as well..
These are the only things that you could do. Or, you could do a search for the width of the screen in that function and replace the resizing code, thus making the images fill the screen. You could take the code from the resizeProjectFull() function. So keep the FIT option for screens larger than mobile, then change the replacing technique with filling, or anything else.
Experiment with these. But please note that i don't offer help for custom work, so i cannot give any more assistance than what i already wrote.