 FACT: The size and heaviness of a 300lb adult human is greatly  exaggerated throughout the movie. The clothing Rosemary holds up is  far too large for her body, chairs and benches would not collapse  under 300lbs of weight (a two seater bench is obviously meant to  hold at least 400lbs), and a car that previously held three chubby  gentlemen (Hal, Leevoy, Mauricio) and one average sized gentleman  (Ralph) wouldnt collapse under 300lbs of weight.