It's even strange that this package is in extras-devel. The autobuilder announced that it would remove the package and not queue it, because it is contained in Nokia and/or SDK binaries. I removed the package from the control file afterwards and reuploaded the sources. Probably the size mismatch comes from different builds.

As an immediate fix, try using libx11-data (= 1.1.99.6+0m5) | libx11-data (>> 1.1.99.6+0m5+withxcb) as dependency (Edit: looked up the correct string, use ">>" instead of simply ">", as this actually means ">=", Edited again: removed the 2: and additional libx11-data).