As others have said, details make the difference. JPG
compresses by dropping out data, which is why over
processing shows little rectangles. I just saved a 1020
x 768 JPG at compression level 8 and it's under 5 KB.
But it's just a solid blue field, so it's easy to store that
data as something that boils down to "783360 instances
of pixels of color 0, 0, 256". Interestingly, when I opened
the JPG in a hex editor it turns out that most of the
5 KB is just null byte filler. The "start of stream" marker
that indicates the beginning of the image data is two
bytes, FF DA. The actual image data seems to be only
17 or 18 bytes.