As the largest consumer of energy is the display of a smartphone, you don't need the same battery size to get the same runtime in a smaller phone. Also by increasing the depth of the smartphone by just 1-2 millimeters you can offset the smaller area available for the battery.
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.
So make it 3mm thicker?
What do you mean by newer iPhones? They use stainless steel since 2017 in the models X, XS (Max), 11 Pro (Max), 12 Pro (Max) and 13 Pro (Max), so it's nothing new.
The iphone 13 is 7.65mm. The Unihertz Jelly is 16.5.
Apple hasn’t made a phone thicker than 10mm since the 3GS, and that was 12.3 (up from the original 2G’s 11.6 because of the rounded plastic back vs flat aluminum).
- people tend to correlate size and price, and by default the correlation is direct (for some things it's inverse), so at similar capabilities (and thus prices) consumers will tend to go with the larger version
- for a smartphone specifically, there's a direct relationship between battery size and device size, and battery life is a really valuable convenience
The iPhone 13 mini has a 2400 mAh battery, the 13 has 3200. 33% more battery capacity is a lot, and at 2400mAh I don't think the mini doesn't survive an entire day of relatively heavy use without a charge.