It sucks that there's very few reviewers actually doing 1:1 comparing of battery life. I don't know how you use your phone, so benchmarking the phones against similar tasks (Wi-Fi browsing, games, etc.) is the only useful way for me to see what's actually going on.
I do think it was a mistake not to make the mini slightly thicker than its cousins (something like 0.31 or 0.32 inches in depth instead of the same 0.29, since as we've seen with other recent Apple products a fairly negligible increase in depth can yield some disproportionate battery gains) but we'll see how the market reacts. I'm hopeful this phone size sticks around when I'm ready to upgrade.
I recommend watching Dave Lee's video about iPhone 12 mini - he tries to make battery life benchmarking with some interesting results (for example showing how light/dark textures are important for battery life in phones with OLED screens):