when you view an image, sometimes it's not in it's original size so there's an option above the image to view the original size. if it's clicked there's an option to always view the images in original size (i forgot the exact name of the option) so i enabled it.

now when i view an image, it's always in full size but it is too large so i have to zoom out every time.

is it possible to disable this option?

There is currently no way to do this from within Safebooru, however it can be undone by clearing your browser cookies. You may be able to access a list of your cookies and just delete the data for safebooru so that you don't have to delete everything, but the method of doing that will differ for each browser.