Image resizing in the admin area is for uploaded images which has the source image resized before being saved to disk.
What you are seeing here is image sizing using CSS using the following code in the forums CSS file:-
.msgBody img, .PMmsgBody img {
max-width: 750px;
height: auto;
}
Most modern browsers support this and it works very well so that the image is not too large for the post area.
However, support is not so great in IE 9 where the image is not auto resized on the height and sometimes makes the image proportions incorrect.