I think you are misunderstanding the meaning of the width component (e.g. the 10 in your example) of a format specification.  As described in the documentation, width is a decimal integer defining the *minimum* field width, not the *maximum* field width.  As necessary, the generated field will be as long as necessary to represent the item as requested by the format spec, but it will be at least "width" characters long.

http://docs.python.org/3.3/library/string.html#format-specification-mini-language