dormmy's realm is larger than mephisto's and at his peak d>m imo. but outside the dd his power is less and in hell m is at his peak. neutral space d wins.
when d tried to conquer mephisto's hell (the largest of the earth-related hells), he needed to use a trick--he needed satannish to ALSO attack mephisto. satannish (who was supposed to be an ally of mephisto) surprise-attack m and dormmy took control of hell for a short time until m joined with the other 'hell lords' (hellstrom, pluto, hela) to cast a spell to boot him out. m personally killed satannish as dormmy took off.
at no time did m and d actually face each other in battle, though m wanted to badly and, as i said, d used trickery to gain hell in the first place, rather than face him. in another series, d was shown to be a splinter realm lord, just like mephisto.
like i said--at his peak d>m but not so much greater as to be able to take mephisto out in hell, imo.