We know there were giant creatures roaming the Earth millions of years ago. The atmosphere was able to support such creatures allowing their blood vessels to pump more blood throughout their bodies and thus helping their bodies to grow massive. As such, much of their food (in particular plant food) was really big too. I have never heard of giant mushrooms though, other than in science fiction stories.
http://www.eartharchives.org/articles/when-giant-mushrooms-ruled-the-earth/
