It's said by the painter Andrea del Sarto (who was a real person--1486-1531), in Robert Browning's 1855 poem by that name. You'll recognize another well-known line a little later in the same poem. Here's how Browning had Andrea del Sarto say "less is more." He's addressing his beautiful, but somewhat stupid and apparently unfaithful young wife, Lucrezia, for whom he abandoned an important painting commission and--some have said--his true calling.
...I could count twenty such ...
Who strive ...
To paint a little thing like that you smeared
Carelessly passing with your robes afloat--
Yet do much less ... --so much less!
Well, less is more, Lucrezia: I am judged.
There burns a truer light of God in them,
In their vexed beating stuffed and stopped-up brain,
Heart, or whate'er else, than goes on to prompt
This low-pulsed forthright craftsman's hand of mine.
Their works drop groundward, but themselves, I know,
Reach many a time a heaven that's shut to me,
Enter and take their place there sure enough,
Though they come back and cannot tell the world.
... Somebody remarks
Morello's outline there is wrongly traced,
His hue mistaken; what of that? or else,
Rightly traced and well ordered; what of that?
Speak as they please, what does the mountain care?
Ah, but a man's reach should exceed his grasp,
Or what's a heaven for? ...
