What determins button image size?

Is icon size is arbitrary? sometimes the icon is small, sometimes big, even if it's the same icon from the same exe.
Here is an example with "New button" what is making one icon bigger than the other?
I would like all icons to be the same size in every custom button.

The second New Button must have the Large Icon option turned on.

Oh, you mean the very obvious "Large image size" wow. thank you.