The phrase 'less is more' was first popularised by a German architect, Ludwig Mies van der Rohe (1886-1969).
He used this phrase to describe how he thought buildings should be constructed. Namely that the building should be stripped of any extras and that it should be constructed on essentials which would lead to clarity, utility and effect.
He was disinterested (unlike most architects) in the purpose which would be served by a building. He felt that if a building were constructed simply, then it would be pure and open and thereby able to serve any function.
This led to many buildings being built in quite a stark and almost 'brutal' fashion, whereby they were built of very stark materials and did not have any architectural features or anything to 'soften' the impact of the building on its environment.
This led eventaully to the 'brutalist' form of architecture emerging, which enjoyed its heyday during the 1950's and 60's, but is laregly out of vogue currently.