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?
Best Answer
Your interpretation is right
The enlarge ability is a magical ability, but it doesn't mention the necessity of maintaining concentration, neither does it refer to any spell (you might think of "Enlarge/Reduce", which would require concentration). So all requirements and all effects of this ability are mentioned in the action.
It does not require concentration or components (verbal, somatic or materials). The ability lasts one minute and can only used on himself.