Cell Size Statistics...

Cell Size Statistics gives statistics for cells within the current notebook.

Total Bytes

Includes overhead for cell data structures.

Content Bytes

Gives the size of text data only. This corresponds to a character count of the text.

Reflect Sharing

Specifies whether information that is shared by more than one cell should be counted. Mathematica stores only one copy of a given token (variable name, and so on), even if it is used many times. Therefore, it is not possible to assign the memory used by shared objects to any particular cell. If "Reflect Sharing" is off, all memory will be counted as if it were not shared, which will over-count the actual memory being used. If "Reflect Sharing" is on, each cell will be assigned the memory used by the token divided by the number of times the token is shared.