Why are some modern art paintings so gigantic at the museums?
It depends which art form you’re talking about, if I assume you’re talking about the 60s Modern American art movement there’s still a range of different reasons. An easy one is that Pop Art usually portrays things on such a large scale as a reference to commercial art forms (e.g. billboards). But I’d say generally it is according to the artist’s tastes and what they think is the best way for their art to create impact. For example Mark Rothko was a majorly picky bastard about the viewing of his art, he wanted people to stand something like 16 inches (totally a guess I can’t remember) from the painting and in dim light to view his paintings.
