Thats pretty big, but bigger has come out of my tank. I had one a couple months ago the size of one of those disgusting mini oranges, cant think of what there called. I have been fighting it for over a year now, its finially "under control" but I pull out grape size or bigger on an almost weekly basis.
Come to think of it. I had at one time some grape calupra in my fuge...could be this is a deformed piece. I allowed it to dry out and it was pretty cool to watch it darken up inside with dead spores.