Window sizes can vary widely and if you come from the same IP with the same exact window size (1033x832 for example) then people wanting to track you for ads etc will have a higher degree of confidence that you're the same person. It's part of "browser fingerprinting", which can also include things like the extensions you have installed: https://amiunique.org/
Tracking/advertising corporations have developed techniques called 'browser fingerprinting' where innocuous seeming things like screen size and the fonts you ahve installed on your system can be used to uniquely identify you and track you across the internet even without cookies or anything like that.