resizing to the size of the original isn't necessary. your crop seems to contain about 1/3rd of the original image horizontally and vertically, so that would make the cropped file (without resizing) around 1440 by 1080 pixels. If you would now post / view this file as a 1440 by 1080 picture, so that each pixel in the file is represented by 1 pixel on the screen, that's a 100% crop.
If you would size your crop up to the 4320 x 3240 size of the original photo, then every pixel in the file would be represented by 9 pixels on the screen (3 horizontal by 3 vertical), which would be pointless since it's not getting you any additional detail.
Also, posting / viewing the 4320 x 3240 pixel file as a 4320 x 3240 photo would be a 100% view (which would almost certainly require scrolling around to see all of the image, unless you have some ridiculously high-res screen).