My large puppy sits on smaller ones!

[StillMedusa]

She definitely wants to play..it's all happy waggy tail and bounce, but she is a big dog.
My brother has visited this weekend with his 7 month old small breed and while they were fine when we were out off lead at the park..lots of chasing and fun, in the house she kept trying to sit on him ..and she is more than twice his size.
She's just coming up to 16 weeks and is a 13kg enthusiastic, uncoordinated ball of fuzz. We are starting training classes next weekend but I would just like to ask how to discourage this.. she did it to a random small puppy out on a walk once before but this weekend has been ridiculous. I had to put her behind the stairgate to give brother's dog a break!
I don't think it's unfriendly in any way..she is desperate for him to play with her but he is quite timid.