Dynon Avionics manufactured "Normal brightness" and "Super-Bright" versions of the EFIS-D100 and EMS-D120 (and also the FlightDEK-D180). Several years ago, we discontinued the "Normal brightness" versions. In this case, it's likely the EFIS-D100 has a Super-Bright display, and the EMS-D120 has a normal brightness display. You can tell the difference by the P/N on the back of the unit. If the P/N ends in -000, it was manufactured with a normal brightness display. If the P/N ends in -001, it was manufactured with a Super Bright display.
An upgrade from normal brightness to Super-Bright is still available - the cost is $400 (plus return shipping).
If the "full brightness" of the EMS has reduced over time, the likeliest explanation is that one of the backlighting components has diminished its full brightness, or failed.
Contact Dynon Avionics Technical Support - support AT dynonavionics DOT com to arrange for an upgrade or repair.
I am familiar with this aircraft and problem. The EMS is at about 60% of the efis. You can dim and brighten on either but the EMS no longer goes as bright. Aircraft 5 years old 160hrs.
It is not on auto dim but if you switch on auto dim it does not change situation.