Why does a custom UIButton image does not resize in Interface Builder?

Why does a custom UIButton image not resize with the button?

I set its view mode to Scale to Fill in Interface Builder, but unlike a UIImageView image, it doesn't respect that setting.

Am I looking in the wrong place or is it not possible?

While this may not be quite what you're after - the background image does scale.

Try this:

[button setImage:image forState:UIControlStateNormal];
button.contentVerticalAlignment = UIControlContentVerticalAlignmentFill;
button.contentHorizontalAlignment = UIControlContentHorizontalAlignmentFill;

In Swift:

button.contentVerticalAlignment = .fill
button.contentHorizontalAlignment = .fill

Remember to set Control Alignment settings for the button