I assume you meant the preview in the "test area". That is a real preview of the cursor as it would look like when used in Windows. Windows always rescales the cursor to its default size and uses low quality rescaling method (simply leaving out pixels).
It is a good idea to design the cursors in high resolution, but before actually using and publishing them, you should use the editor to get them back to the standard size. (It uses a better scaling method than Windows, because speed is not an issue at this point.) If you want a quick preview, rescale them, look at the test area and then hit Undo to get back to designing them at high resolution.