Jack (1 year old, ~10 lbs) and Rizzo (4 mos. old, ~4 lbs.) get along well,
and pass many hours of the day engaging in vigorous competitions from the
Kitty Olympics: chasing, wrestling, climbing, et cetera.  During their
quieter moments, Jack likes to groom Rizzo with what appears to be great
enthusiasm.  If Rizzo is sleeping, he'll often just lay there and take the
grooming onslaught.  If he wakes, and I'm nearby, he'll give me a look that
seems to say "can't you get this big oaf off of me?"  If it persists, or
Rizzo is wide awake, he may try to get away from Jack.  Jack then pins him
down with a big paw and grooms Rizzo all over.  Rizzo will struggle and
flail, even go so far as to rabbit-kick Jack in the head with his back feet;
none of this deters Jack.  It often devolves into a full-bore wrestling
match from there.

Rizzo isn't frightened of Jack, and will often groom him as well.  Neither
expresses any verbal distress during these incidents, and neither has ever
injured the other.  The only reason that I can see why Rizzo might find is
uncomfortable is that Jack tends to groom against the grain of Rizzo's fur.

My question is: is Jack grooming simple friendliness, taken beyond which
Rizzo finds friendly?  Or his he, in a big-brotherish kind of way, trying to
show Rizzo who's boss?