Riku's both taller and older. Plus, he has a different tendency for clinginess than Sora. Sora's the one who lets himself be comforted, Riku less so. So I don't mind it when Riku's the one wearing the pants.
Granted, unless it's crack, Sora is still a guy and should behave appropriately. But I can see him leaning against Riku for a good cuddle >3>