Had this happen to me once with an iPhone 14 Pro. Worked normal when I plugged it in the next night because I can normally go about 2 days per charge.

Probably a software bug with eco charging or whatever. I turned that setting off after experiencing this.

Realistically the pill cannot get smaller. The area around the pill is already enlarged somewhat most of the time.

If the pill was shrunk it would reduce the functionality of it. I expect the pill to be around until they can hide everything under the screen.