A plastic, storage bag would not weigh more when it when it was filled with air, but when I blew up this balloon, its weight did increase. Why?
The difference is that the balloon has been inflated enough to stretch the rubber. That compresses the air inside the balloon, making it denser than the surrounding air. Because it is denser, its weight is not balanced by the push of the air it displaces. The more compressed the air in the balloon is, the more weight the inflated balloon will gain.