Photomerging RAW files

Hi,

I am trying to create a large picture file from high quality raw files. I'm
trying to create something similar to a panoramic (e.g. 1 photo x 5 photos) but
instead am trying to do it two dimensions (e.g. 5 photos x 5 photos). I have
managed to create a large '25 picture' photomerge although when I look at the
picture quality at an actual size pixel level, the original raw files have
substantially more detail compared to the photomerged picture. The difference
between the original files and the new photomerge is really different. Does
anyone know what I'm doing wrong? I'm using the 'Photomerge Panorama' in PSE 6
for Mac. Is there a limit to the resolution in Photomerge mode?