Font size changes in Excel 2003 charts when re-opened

For certain charts that I create within Excel 2003 (usually bar
charts), I use a font size of 8 (eg for data labels, axis labels, and
scale). If I close the file and then re-open it, all these fonts will
have become size one (title and legend are not effected).
Oddly, if I have created a bar chart and made several copies of it
within a worksheet, it will only be the original and not the copies
that are affected by this.
If anyone can shed any light on why this may be happening and how it
could be fixed, I would really appreciate it.

Lucie