My friend DC's puppy Rex:
Parker met Rex last week. Rex greeted Parker with lots and lots of barking. Parker, who for the moment outweighs Rex by 10 kg (but won't for very long) looked at Rex, blinked, and, with Rex following and barking hysterically, proceeded to investigate the apartment. Within about an hour Rex had decided that Parker was really cool and Parker had decided...well, nothing, because Parker is 4 and Rex is about 5 months old, so Parker just ignored Rex.
They'll have a play-date again when the weather cools off.