[Hebrews 11:6]

If you specify width=500, you're producing an image exactly 500 pixels wide. A full computer screen is about 650. But many are using smaller screens, on phones and tablets, and bigger images get cropped.

Using "50%" or 90% avoids that by tailoring your image to every screen. It does not "depend on" the picture's width; rather, it fits the picture to the viewers' screens.

[Kevmo]

Good to know. width=50%

Note that in Preview, this picture is still gigantic...