The mini could have been simply, iPhone. The marketing would have been that they managed to add an extra .7" of screen, while reducing the overall size and weight. That's a great pitch. Who doesn't want a bigger screen in something that more easily fits in their pocket? Instead they called in a "mini", people thought it would be tiny and hard to use, so they didn't buy it.
The iPhone 12 mini screen was only .1" smaller than the screen on the iPhone 8 Plus... the giant option from just a few years earlier.
The mini was a marketing and brand strategy failure, plain and simple. It wasn't a small phone.