There are a ton of these old weight-gain ads for women from the 20s and 30s. Typically they appeared in magazines for young women and in fitness publications. What's interesting is that a swell in breast size was usually incidental to overall filling-out. So the skinny girl would fill out in the shoulders and hips as well as experiencing increased breast development.
The ad at right is for a product called "Kelp-A-Malt", though "Ironized Yeast" more commonly used this sort of tableau. In this one, the target customer isn't meant to merely fear that her lack of curves and small breasts might cost her success with men - which is the most common approach - but also that inferior, flatter-chested women will learn the secret of growing larger breasts and will leave her behind. It's particularly nasty - start buying Kelp-A-Malt today or your friends will all have bigger boobs than you!
I got this one from Modern Mechanix. It's from the November, 1934 issue of Physical Culture.