Still using my 13 Pro here. Coming from an iPhone 12 Pro Max. Battery life for me is still great; the 13 Pro outlasts my 12 Pro Max in my daily use.
Reasons for downsizing: easier handling, less weight in gym shorts, no compromises on features.
Does the 13 Pro Max have better battery life? Yes. But I don't see the value in having 50-60% battery left at the end of the day before I plug in. Clearly the power reserves vastly exceed what I use my phone for. So it's pointless for me to upgrade to the 13 Pro Max. I'd rather have a phone that (for me) is more manageable while I'm on the go.
Something to consider, and this should NOT be considered over ergonomics.....because these things aren't worth inconveniencing yourself over the life the devices, more nitpicking than anything.
One of the main factors with battery longevity is going to be total battery cycles. So 100% to 0% = 1 cycle. 100% to 50% = 1/2 a cycle. A larger battery will experience less cycles over the course of its life (assuming the same usage) meaning it will have a longer service life.
Another factor is depth of discharge or more accurately the range of charge/discharge. The narrower the range of its voltage the less stress is placed on the battery. In theory keeping a lithium ion battery between 3.2 and 3.8 volt (~30-70%) charged can add significant life to it. While this isn't worth the time to manually maintain (Apple does some of it with "Optimized Charging") its worth noting staying within a narrower window is easier with a larger battery because a narrower range last longer with a larger battery (more energy per %).
Both of those will contribute to maintaining what Apple calls "Peak Performance Capability" mode. As the battery ages a heavy load will cause the voltage to dip more and more due to higher resistance (think of starting a car on a cold day with a weak battery and the dome light dimming). Once the voltage drops below its shut off threshold, even momentarily the device will shut off. Apple will then throttle performance of the SoC to prevent these loads so your device doesn't randomly shut off.
Lastly is considering future versions of iOS. The older the device the more demanding newer software will be on the hardware. While both devices will see a hit to battery performance with time the larger the battery might be enough to get you through the day while the other may not (theoretical)
However here is some actual data using extreme cases.....
My iPhone 11 Pro Max - purchased early 2020
My iPad Air 2 - purchased early 2015
Even those the iPad's battery is 5 years older and has had to discharge nearly 4x the amount of energy (cycles x mAh quick dirty math) vs the iPhone their current health is on par with each other. Yes there are caveats like 85% being a portable device nominal charge and charging conditions with the iPad being more hospitable.
Now before everyone starts flaming me, I wouldn't buy an unwieldy device because the battery is bigger. The device needs to work with your preferences and lifestyle above all else. However all other variables aside (include financial) and I need one reason to buy one device over another, larger battery would be my choice all day regardless if the small battery was "enough" for my use.
Hopefully no one takes this as a jab toward the iPhone 13 Pro because its definitely not, I'm sure its a fantastic device.