Viewers default to a size which is appropriate for providing a useful view of a file’s contents, but small enough to stay out of the way while you work in the windows which own them. Unless you resize it manually, a Viewer will resize itself automatically when the file it’s displaying changes.

The exact size used for a Viewer is determined by an algorithm that considers your screen size, the type of data being displayed in the viewer, the size of the image in the case of image files, and the viewer size setting in the Viewer panel of the Preferences window.