M-ATX boards and cases are like compact and can be carried around. Usually weighing between like 7~12 pounds or so. While Full size ATX cases can go all the way up to 50 pounds and break your back whenever you try picking it up.
Currently I got 2 Micro Atx's and 1 Full Size Atx. In all honesty, unless your grandpa is going to have a HUGE GPU and a big CPU cooler. It would make more sense to get a Micro Atx case and mother board. Besides smaller cases and boards are easier to deal with, unlike the damned ATX cases and boards.