You may have noticed that GAME doubles every graphic's size, whilst GAME_FMF does not. In fact, this scaling is normally a good thing, since artistic details on the pixel level can disappear to someone who isn't scrutinizing your graphics (i.e., the average player). A mobile phone, however, has a very limiting display; hence, graphics are not doubled in size.
A notable exception to this is the hero's picture in the menu, which is doubled using the [HQ2X] algorithm. This can be done real-time, but this grinds the game to a halt. So, the HERO_X.png lumps are used to store natively the results of the scaling algorithm.
Here's an example of the kind of clarity HQ2X adds: