1. Cheaper iPhone
2. You don’t want to carry a big iPhone and you have a Mac/iPad nearby anyway for anything complicated so a big phone is unnecessary.
The second case should motivate wanting eg their ‘pro’ cameras in the small phone but the first case motivates making it cheap and low-margin. You can also imagine a world where apple markets a mini phone as also being an optional companion to a bigger phone, but they already have the watch for that.
Wasn't the 11 pro the size of the mini / SE? I don't remember whether there was an 11 mini, but it seems to me that 11 pro / SE were exactly that, and the current minis / se are the size of the 7/8/se/11 pro.
I the time I couldn't justify buying a new phone, but I remember late last year, when I figured I could start looking, I was possibly contemplating getting the 13 pro, for the camera. But when I saw how huge it was, I immediately abandoned the idea.
The iphone 12 mini and 13 mini are both smaller than the 2nd/3rd gen SE but larger than the 1st gen SE.
The 2nd/3rd gen SE is the same size as the iphone 8.