File size on disk is not the same as file size in memory.
Finder shows you the file size on disk which the file takes up after compression. The jpeg file format is very good at reducing the storage requirement of a file and vastly reducing that size, albeit at the expense of losing some image information and adding some artifacts every time you resave it..
Image size shows the size, in bytes of memory, of a single layer and is a simple calculation of the number of pixels (H x W) x 3 for 8 bits /channel images or (H x W) x 6 for 16 bits/channel images. Note 1 Megabyte = 1048576 bytes