How does WM compress weight maps?

Hi, i needed to edit some of the weight maps in Photoshop, i then exported them to 8bit grey scale, however they are still double the total bits and file size when i check the details of the image in windows. :?

This is the original image from WM. https://image.prntscr.com/image/KvYWn0U8QUazI_2RpziRgw.png

and this is the edited one from PS, exported at 8bit greyscale. https://image.prntscr.com/image/u1Xo-J4ESea5CtI-Kp0mTg.png

No one?