File size in kB or MB has nothing to do with image size in pixels.
What you need to understand is that the jpeg format uses very aggressive file compression, while what you see in Photoshop is the uncompressed expanded size in memory. Jpeg compression is extremely effective. I just saved a file as a flat 8-bit TIFF at 90MB. Saved as quality 8 jpeg the same file weighs in at 1.75MB. That's 1 point 75, or about 2 percent of the original size.
Unfortunately the compression is also destructive, non-reversible and cumulative. A jpeg repeatedly resaved will eventually turn to mush and disintegrate.