It is possible to mix too much in a mechanical mixer but you would have to
mix on high speed for a long time to break down the gluten structure.
One reason for your bread growing slower may be that processors put a lot
of heat into the dough and processor doughs often end up warmer than their
mixer counterparts. Try using water at a higher temperature (say 10 C
higher) to mix the dough and see if that makes a difference.
John Wright
Yorkshire, England
"That which does not kill us makes us stronger"