changing resolution is totally different than changing dpi in build.prop which isn't hardware at all its just for the android UI as i said before, and i know that the icons are pre rendered but the size of them depends on the DPI, they will "APPEAR"smaller depending on size of screen try changing DPI in build.prop and you can definitely see what am telling you (same thing when you lower resolution on your PC doesn't the icons which are pre-rendered appear bigger?)
changing the screen resolution on other hand can't be done in build.prop it needs an app like
This one which changes the resolution of screen and therefore lowering the DPI EFFECTIVELY, while changing DPI in build.prop only makes Android OS draw in lower DPI but the GPU still outputs the native screen DPI since the resolution in still the same hope you got the point in difference.
in both cases you need root access, try both and you will see a lot of difference
The DPI in the build.prop file refers to the screen hardware
the HARDWARE screen DPI is fixed in manufacturing can't be changed at all.