Misunderstood circular sector patch antenna results

Hi all
I am simulating a circular sector patch antenna in cst mws; it is excited by a waveguide port defined at the beginning of a microstrip line (see image)sectorial_patch.bmp

Now what I can't understand is why if I reduce the dimensions of the substrate and ground plane, not only the total efficiency, but also the radiation efficiency fall down. In particular, the reduction in radiation efficiency is higher if I reduce the length of the microstrip line.
Is this related to the radiation properties of my circular sector antenna or to an imperfect transition between the waveguide port and the microstrip line?

Thanks anyone could help me

Sorry,
I forgot to say that the software I'm using is CST MWS, if that could help.