Because they saw that photo of Marilyn Monroe on the beach when she was in a bathing suit while pregnant and pretend she was chubby. And they reeeee that all pin-up girls look like that! Edit: https://www.google.com/search?q=marilyn+monroe+at+the+beach&oq=marilyn+monroe+at+&aqs=chrome.1.69i57j0l3.7234j0j7&client=ms-android-google&sourceid=chrome-mobile&ie=UTF-8#imgrc=mNwn4MJ7Q0Y_AM: