Not that I want to diminish Twilights role, but I just can't imagine that she's the first gay unicorn with a ton of magical prowess thinking to herself "Oh how lovely it would be to just cast a spell and get her belly to grow twice the size". Maybe she just reinvented it? Rediscovered it? Or perfected an older version that was forgotten due to minor side effects?
Basically the end bit of what you said. There are similar spells that Twi will make functional.