John Gill Commentary 2 Kings 3:18

"And this is but a light thing in the sight of Jehovah: he will also deliver the Moabites into your hand." — 2 Kings 3:18 (ASV)

And this is but a light thing in the sight of the Lord
To give them such a plenty of water in such an extraordinary manner: he would do for them what was greater, not only save them from falling into the hand of Moab, which they feared,

but he will deliver the Moabites into your hands ;
which was more than was asked for, or expected.