@stanlyF already quoted how the font size is meant to be interpreted. Especially observe that it is about the nominal height of lines
You seem to have the misconception that it is the widths. As already mentioned in my answer to your former question, the widths of a string depends on
- the font metrics, especially the character widths of the characters involved,
- the font size,
- the current character spacing value,
- the current word spacing value,
- the horizontal scaling,
- the current text matrix, and
- the current transformation matrix.
For details read the specification.