The camera size is already huge in iPhone 14 Pro that it doesn’t sit well on MagSafe Duo Charger wishing that they figured out a way to at least keep the size to how it currently is but it looks like there’s no getting around it. It looks like it’s gonna be bigger than 45mm Apple Watch at this point.
If they filled that space with battery, it’d probably be closer to 400g. Batteries are very heavy.
Battery size is obviously a compromise. Apple would have tested larger and smaller batteries, and come to the conclusion that the current size is perfect. They might change their mind, since battery life largely depends on what apps the user runs and trends change over time, but they would’t be far off the mark.
“Something else” would need to be non-conductive (to be compatible with wireless charging) and easily recycled (to meet their environmental commitments). Glass is probably the best option.
Also - glass isn’t as heavy as you seem to think. Plastic is only lighter than glass if it’s really thin, and thin plastic is too bendy. The back has to be stiff enough to stop the phone from bending, otherwise the screen will crack… plastic would be very thick (and very heavy) to achieve that.
The early model iPhones with a plastic back all had steel reinforced plastic backs. Steel is heavier than glass. And it blocks wireless charging.