I don't think this is true. I've never been given a true option about batteries.

It's not like I get to choose two different versions of the same model of phone, where $a has a bigger battery, and $b has $feature. It's usually a funky mismatch between two different models, with different specs, some of which make it faster, and some of which have a bigger battery.

But I've never seen phones with a 1-to-1 comparison for batteries.