You can severely hamper the visitor experience on various devices by using larger size images when they can easily be converted and made a fraction of their size.
The discussion is mainly focused on network packets, but even before network packets, you must consider disk cluster size. If a disk cluster size is 4kB, you should consider resizing your images your images in multiples of 4kB. This advice is not helpful if your image 1kB to begin with, but if we’re talking 10kB, reducing to 8kB would only require two cluster reads versus three cluster reads for a 10kB.
This is just another consideration when you are considering scalability of your web server.