The notion that simplicity and clarity lead to good design.
This is a 19th century proverbial phrase. It is first found in print in Andrea del Sarto, 1855, a poem by Robert Browning:

Who strive – you don’t know how the others strive
To paint a little thing like that you smeared
Carelessly passing with your robes afloat,-
Yet do much less, so much less, Someone says,
(I know his name, no matter) – so much less!
Well, less is more, Lucrezia.