power gallery creates some resized images that are cropped and others that are resized proportionally.
see this thread for details:
showpost.php?p=36751&postcount=2
in the gallery index.php file, there is a line similar to:
$gallery = get_gallery(get_mediarss_url('gallery=' . $pg_id . '&thumb=62c&full=782c'));
this is setting it to use the 62c image for the thumbnail and the 782c image for the large one.
you can change that to:
$gallery = get_gallery(get_mediarss_url('gallery=' . $pg_id . '&thumb=62c&full=782p'));
to set it to use the proportional image instead of the cropped image.