If you press the i key several times in Develop or Library-Loupe view, what are the pixel dimensions of the image—on the same Loupe-info view as the date/time are shown, before exporting? Is your camera a 24 megapixel camera or a 66 megapixel camera?
To explain why your image is exporting the size it is, can you show us a screen-capture of your Export panel with the Image Sizing section opened to show the various updates you have set, or if you cannot figure out how to do a screen capture, at least list each of the parameters and their values you have set.