Are we talking about all horses here or only qhs?
If we are talking qh's I can't really judge because I've never had a whole lot to do with the breed. But I have done with other breeds. I've always had largish horses but the biggest one is my wb who is now 17.3hh. He's tall and chunky but he certainly knows where everything is and what we is doing. He does have his moments where he is more pushy and seems less intelligent but I think all horses have those moments its just worse when a huge horse does it :)
IME I would have to say that I don't believe them to be this way but once again I don't know if we are only talking qh's here or not :)