I was also hoping to find another longer story, and hoping if it rings any bells for anyone. This one is much older and most likely not on this site.
I believe the story was about some sort of serum or shot, and it had inadvertently caused an unsuspecting boy or scientist to regress. There might have been some gender changing involved, similar to the story “French Whines”. Eventually, news of this serum captures the interest of a rival group, or some thugs who want it for whatever reason. The main story conflict I believe is that our characters are being chased/hunted by these thugs. The clearest part I remember is that near the end, the thugs are redeemed. The bigger one, a large manly woman, gets a dose of the serum - but it’s a modified version, and it will regress her back to a baby, but she will grow up to become cute, cherubic and petite. “Like an angel”. Like the ugly duckling who grows up to be a swan, “each year you grow up you will become more and more beautiful and everyone who sees you will adore you”.
That might not be the exact quote, but it was something along those lines.