Eric Portis digs into how the browser decides which image to downloads when you give it <img srcset="" sizes"">. Notably, a browser can do whatever it wants:
Intentionally un-specified behavior lets browsers provide innovative answers to an open-ended question.
Still, calculations happen based on what you give it and you can still do a good job with that part.
The very weirdest part about all this is that the sizes attribute can alter an images "natural width", which can lead to unexpected upscaling, a thing we're trained to loathe. If you're in that situation, you can either...
use an inline width attribute
set an inline style with a max-width
embrace the fact you might experience occassinal upscaling