fungi growing on dead tree

Why is it that when I post photos to DW, they always display as larger than actual size? I have to add a parameter like width="80%" to make them look the same size as they do in my image editor, when I'm displaying them actual size there.
Before I had thought it was browser specific, but now it seems to be happening in all browsers. And I can't tell if other people see the images large or actual sized.
It must be due to me having Windows configured to display things at 125%.