X Server: abused or buggy?

[Óscar Fuentes]

After observing how X used 1.3 GB of RAM, some investigation revealed
that certain application (Okular, a document viewer,
http://okular.kde.org) was causing this memory consumption. It is
creating lots of pixmaps as a way for caching document pages. Opening a
pdf file an scrolling through its pages makes X memory usage grow in the
hundreds of MB. After Okular is closed, usually X returns the memory to
the OS. But not always, sometimes retains some dozens of MB.

Is this considered a bug on the X server, or is it being abused by the
application?

I'm using Kubuntu 8.10, which comes with Xorg server version 1.5.2.

-- 
Oscar