Yes, they did. Exact same dimensions except the thickness (which is fine I'd have preferred a thicker 12 mini with much more battery).

Thing is I bought 12 mini 3 weeks ago (replaced my 4 years old iPhone 7). It was way cheaper than 13 mini at the time of that sale. Also the only thing important for me in 13 mini better than 12 mini was battery which isn't that significant a difference imho.

I hope to use 12 mini for at least 3 more years. Wouldn't want to buy a 13 mini to replace a 12 mini then. I would want a 15 or 16 mini at that time (or equivalent).

Also a pattern is Apple reusing physical specs for a generation or two. The 13 mini is essentially an upgraded 12 mini in the same case. There was no innovation or upgrades in design, so they likely decided to get the most out of their parts stock.

Some think the 12/13 is the last small apple phone. And like the OP implies, it's a catch 22: They only put non-premium specs on the small phone, so people don't buy them, so they justify killing it because of the size.