my girls are both lab mixes and were pretty close to their final weight at 6 months...the have each gained about 10-15lbs since (50 then/65 now). Most the purebred labs I know are lcoser to 75 lbs...I would guess if the tempernments of the two dogs is right, that will play a bigger part then the size you your dogs. My dogs play as well with dogs under 25 lbs as they do with dogs over 100!