I assume that when PS opens from LR it is just the Restore Down size (the button you have when a window has been Maximized), and that you are able to Maximize the window and everything is fine, that this is just a nuisance. I also assume that when you say opening Photoshop alone it comes up in full screen, you just mean Maximized.
With that in mind, this shouldn't be an LR or PS bug, because I haven't heard of anybody else running into similar behavior. However, there could be some sort of interference on your system.
The first thing I would suggest is to create a new user account and test there. This will determine if some setting or other is the root of this behavior.
