Yes, canvas tents are made of tightly woven cotton. As the canvas gets wet it swells and tightens even more. And yes they are heavy. Definitely not for backpacking, although as a kid I would carry half a military pup tent and use it as a lean to and get my heat from the campfire.
This is the tent my son recently bought for elk hunting. The stove jack is on the far side. That front porch was sure a nice addition to the wall tents of old. This monster can hold six cots at the back, with room for a table and chairs at the front. Great when that early Colorado or Idaho snow storm sets in. Here it is set up on the beach at Flaming Gorge on the Wyoming side.