Monroe was not plump. She was utterly tiny most of her life, ranging from a British size 8 to a 10, which would translate to a US size 4/6 to 8/10.
Her dressmaker had the measurements of Monroe's waist being 22 to 23 inches, and her hips at 35 to 36 inches and her bust at 35 to 37. While this is wonderfully hourglass, a waist at 22 inches is
incredibly small. I have only had a waist this size when I was 21, and weighed about 105 pounds.
Interestingly, Monroe had a small waist than a lot of the current VS angels. Again, she had nearly the same measurements (albeit a slightly larger bust and hip and smaller waist) to Miranda Kerr or Adriana Lima. The difference, of course, is the height. She was about five inches shorter.
I suppose you could say VS angels are kinda *stretched* Monroes

.
I will say though, that it was a hellova lot easier to be slim back in the 50s and 60s because of the lack of extremely processed products in the food chain. I reckon if we were all transported back to 1955, we'd find we all lost weight rather quickly unless we specifically ate loads of cake.