Fog - the filter was heavy because it was full of carbon, and because of the steel structure of the filter. This was a big, self-contained, filter unit, designed to fit into an even larger filter holder built into the ventillation/ducting sytem.

But think about it - that filter is used with either a 24" or 18" (the size of the inside hole on the filter) duct. 24" duct and 18" duct are the size duct you see hanging from the ceiling of the Home Depot. It's huge! You're talking about venting some serious cubic footage with a filter that size.