Correct. This is the purpose of the whos command. It shows the memory used for storing a variable. This is the number of elements multiplied by the number of bytes per element. For cells this is the number of elements multiplied by the byte size of a pointer.
Therefore the size shown by whos is expected to be larger than the number of elements.
If you want to get the size in pixels, use size. If you want the file size, see KSSV's answer.