Unsure what you mean by black lines... mind you, the image I will use as my example is from mobile, so it could be different on PC.
the borders kinda have to be there for some of the menuing imo.
mine way bigger but only on top and bottom of screen it feels so weird when game is almost same size as black on screen :D but maybe its just on my end :/
main menu always had a little of black like this
and now it looks like this ingame