I have found that letting them 3xplore an unknown area together brings them closer, atleast with mine, my ducks are 3 weeks a part, so not a huge difference but enough size difference to cause a prob, my 3 cayuga drakes( 3 weeks older, and triple the size) were horrible with my girls at first, chasing them,biting them, just being bullies, I actually caught one of my cayugas dragging my blue swedish around by her wing( all is well, no injuries) and their daytime pen had to be separated by chicken wire, so they could see each other all day, then after two weeks I let them explore the yard for the first time together when my girls were 5weeks(cayugas 8), that was it, they sleep together, eat together, swim together...no problems since...i know I could have lucked out, but it definitely worked for me!!! They are pretty much the same size now, the girls got huge when they got outside to free range