I have a copy of an article (somewhere in my files) from Threads years ago on how to make your own mannequin. It involves getting together with another sewing friend, an old Tshirt and either brown wrapping tape or duct tape. with your best undergarments on and that old T you wrap yourself in the tape then cut up the back (where a zip would be) and take it off. Now you have your shape replicated...stuff it, mount it on an old lamp stand and you have your own dress form to help with fitting an designing your own clothes. I bought a dress form years ago at a yard sale (saving $$$!) and she's all foam but covered with a basic cotton sloper cover. I can stick pins in her when fitting...I've changed a little and my clients are all different sizes but she's still great to pin my pieces on when working up a custom order.
This article shows how mannequins are sized, adjusted and altered to suit fashions from different eras. We've had bustle bottoms, corseted bosoms, tiny waists and straight up and down 'boy' shapes over the centuries.
Look at the different busts from this mannequin maker KCI Mannequin. Demonstrating corseted, pre bra, older, younger shapes.
This article is an interesting, useful and fun read....and do go to all the links and go deep.