It is true that it will become larger as it risses in the atmosphere. This is because the pressure surrounding the balloon decreases, so the pressure inside the balloon wants to equalise with the outside pressure. This causes the balloon to expand as the inside air is trying to get out.

That's why things get 'sucked out' of a plane if there if someone opens the door at 30,000 feet, the cabin pressure is trying to equalise by removing the objects and air inside it.

The ballon contains a fixed amount of gas producing internal pressure. At the surface, this pressure equals the surface atmospheric pressure. As the balloon rises, the atmospheric pressure drops, allowing the balloon to expand, keeping the internal pressure and external pressure equal. If the balloon is fully inflated at the surface it will burst at higher altitude.

It is not the sun comes up and in mornin. Morning,evening,north,south,east,west,poles etc are all we subtitued . they are all named by us for our convinience. We assumed that when sun risses it is morning.okay.