Very few others have such powerful enlarging abilities, and it's generally good courtesy not to encroach on another's current territory. So Yasa pretty much has dibs on this entire stretch of shoreline!
I would imagine so! He doesn't seem to be terribly thrilled to be in that position, that seems as good an explanation as any as to why. (that, and I imagine it's quite heavy, despite being quite soft.) She doesn't seem to mind at all, though.