It is quantized, but not necessarily to a power of two. RAM and flash sizes of 3x2^n are common even in contemporary microcontrollers, and it’s clearly possible to use fewer R chips then your bus decoder supports in the less integrated computers of yore.

And once you have a project with a fixed amount of ROM, either because the hardware is at a point in its life cycle where you can’t change it, or it’s third-party hardware and you have to use what you get, or because you’re already at your BoM budget, then your software will behave like an ideal gas - it will expand to fill the available ROM. This happens because until you run out of ROM, you will write your software in whatever way is easiest to get your job done. But then once you run out of ROM, you will go back and look for something that you can make smaller. Then you can add a few more features or whatever, until you run out of space again. This process will repeat until you’re done, but your ROM will always be nearly full.