The smaller battery helped them reach their price point. That’s what Apple engineering is all about now.
No, you don't know what you are talking about, as usual. Do some math. If you add the increased dimensions to accommodate a 5.1" diagonal screen, keeping the same depths of each model, you wind up with approximately 11% more volume for a battery inside. If the battery originally took up only 10% of the original space, then you can easily fit a battery that is twice as big inside a phone that has a screen that is only 0.4" larger on diagonal dimension. THAT is engineering.