The resolution is actually quadrupled by doubling the value of both axes. In this case going from 1500x1424 (2.1MP) to 3504x3327 (11.7MP) multiplies the total number of pixels by 5.4

With the same level of jpeg compression you'd expect it to jump from 700KB to roughly 4MB. Since both images are the same file format, the rest of the file size difference is likely attributable to less jpeg compression being used in the larger image.