Sample grabber media type is not set

Question

My graph is like

video capture filter->sample grabber -> renderer

By default i set

AM_MEDIA_TYPE mt;

mt.subtype = MEDIASUBTYPE_RGB8;

pGrabber->SetMediaType(&mt);

I have called RenderStream and it is working fine.

I have tried to change the grabber resolution  by modifying with below line.Indeed i stopped the graph before changing the resolution and rerun the graph after changing the grabber resolution.There is no affect??????.Still it displays previous resolution.

mt.subtype = MEDIASUBTYPE_RGB24;