In the MM page 122, nothing is mentioned about needing to maintain concentration, and there is no wording to indicate a duergar's Enlarge ability is a spell.
This is in contrast to the duergar's Invisibility action that explicitly says it must maintain concentration.
So, my interpretation is that this is some sort of innate magical ability, not a spell, and thus wouldn't require concentration. Is this about right, or am I missing something?