For anyone wanting to know more about Jayne Mansfield's life, or the history of Hollywood in general, I recommend the podcast You Must Remember This which is fantastic. There's an episode on Mansfield and three on Marilyn Monroe.
And yes, MM was nowhere near a modern size 16. I went to see her clothes on display prior to an auction a couple of years ago and none of them looked larger than a 10 at most.
One of my favourite scenes from The Crown is when HM comes in with her new, shorter, haircut - which she's had ever since. Philip's face, then he asks her if she remembers that she wanted another child
It was a funny scene but it didn't make sense to me because HM's hairdo at the time would have been perceived as young and fashionable, not the old-lady style it is today. Plus I couldn't imagine HM keeping a hairstyle Philip disliked for more than a few weeks, let alone a few decades!