[MrForbes]

Imagine a channel that was 3" tall, with a 1" wide flange on the top and bottom, again 1/8" thick and 36" long. If you started with a flat sheet of aluminum and bent it into the channel shape, it would have to start out being 5" x 36" in size, so it would be quite a bit heavier than just a flat side piece.

And if it were a rectangular tube, it would be even heavier.