JPEGs can be saved with different compression qualities; You can reduce the size of the saved file image file in most any photo editor by selecting a lower quality setting. This is not the same as cropping, as you end up with the same size image -- it just doesn't look quite as good.
For example, I took a digital poto on my 6 megapixel camera recently. An uncompressed TIFF format of the image, 2000 x 3000 pixels, is 17.3 MB; in JPG at the highest quality, it shrank down to 2.5 MB. At medium quality, it was 504KB, and at the lowest quality, it was 192KB. All images were the same physical dimensions: 2000x3000 pixels.
Probably more than you wanted to know...