The mATX board has Realtek® ALC892 onboard sound whereas the bigger board has Realtek® ALC1150
The mATX board doesn't have USB 3.1 but the bigger board does.
The bigger board seems to have an extra fan connector intended for water cooling pumps whereas the mATX board doesn't.
The ATX board has 'Ambient LED' whereas the mATX board doesn't.
The ATX board also comes with a G connector which makes it easier to plug in your front panel connectors, the mATX board doesn't.