The internet went public in about 1991, in 1993 the WWW was published. In these first years of the Internet web design was mayhem. There were no guidelines or regulations of what was good design and what was utter crap.
One of the traits of utter failed web design was that some people, instead of making an image the size it was displayed in, they used some very criticized tags to make the web browser resize the image as they were accessed. At this time speed was of the essence and this became a tiresome time sink on many websites. Another downside is that scaling a picture is still today a precise technique that only a selected few picture viewers successfully manages. Browser will render a resized image as horribly jagged and uneven.
This made the resize argument of the <IMG ...> tag one of the most hated tag attributes.
Therefore I have to ask the WP dev team; why in everything that just and good did you pull this horrible crap out of the attic again? Its quite embarrassing really.
FFS, resizing images IS NOT CLIENT SIDE!!!!!!111111111111 :)