Problem setting to 24p causes smaller picture size for SD movies

Hi,

I have found out after some experimentings how to set up to 24p. Works for HD material
pretty good.
But, since I switched in the XBMC to the 24p settings, my normal SD movies
are now shown cleary visible smaller on the screen.
Looking via beamer, I lose around 10-15cm an all sides.
If I turn of the 24p modus the size is OK as before.

How can I handle that in order that the picture size is the same for HD and SD
on the 24p modus?