As @NicoleHamilton comments, this is mostly a duplicate of a question at Graphic Design. But to summarize its answer, “font size” is a basic property of a font, also known as the height of the font. It is a concept that relates to the font design, not to the height (still less the width) of any particular character, and a font designer may use the font height as he likes.
Technically, the “point size”, more properly called “font size” (which may be measured in different units) only refers to the height of the font. One point equals 1/72 inch by definition by modern principles, but this is just a matter of units.
The font size has no defined relationship to the widths of characters, which may vary a lot.