The battery capacity grows much faster than the display energy consumption, and it's not even a fight: at otherwise equivalent hardware, the larger phone has always had better battery life than the smaller one in every iPhone generation.
The minis both suffered significant criticism due to battery life issues, compared to their larger sibling.
> Also by increasing the depth of the smartphone by just 1-2 millimeters you can offset the smaller area available for the battery.
You can do the same on both smaller and larger form factors so that's not an advantage of the SFF phones.
And much to my dismay Apple remains very much not a fan of that: after having increased the phone depth to long-forgotten heights of 8.3mm (a chonk not seen since the 4S's 9.3), it's been reduced back down to 7.65 in the 13 (up a hair from the 12's 7.4). I fear an eventual return to the dark days of the 6S/7 and their 7.1mm you could shave with (but couldn't pick your phone off of the table for lack of ability to grip the thing without using your fingernails to pry it off).
As an example, the Z5 Compact had a 2700mah battery in basically something ~1mm thicker than a 13 mini, which has a ~2400mah battery. The Z5 Compact is also a 7 year old phone, which didn't have wireless charging.