I used to think this as well but I had an interesting — and surprising — experience that suggested otherwise.
Last spring I was testing a tarp/bivy setup, mostly for testing the bivy which was made for me by a friend. I intentionally went out for a quick overnighter when rain was forecast. The tarp was the HMG Echo II which is a cat cut 8.5'x8.5'x6.5', .74 cuben, weighing about 10.5 oz with guylines. (I weighed it with my digital scale but forgot to record it in geargrams library... however, this is pretty close.)
As hoped, there was a fair bit of rain all night and into the next morning. When I packed up for the short walk out (about an hour) I shook out the very wet tarp fairly vigorously to get rid of as much water as practically possible.
When I got home I weighed the wet tarp on the digital scale and the weight was something like 14.7 oz (again, from memory, but that is
very close).
Yes, this a sample of one. However, IMO it is a very solid data point, enough to convince me that
the weight of Cuben increases by about 40% when wet and that we are not shaking out as much water as we previously assumed. Of course some of this water weight was also in the guy lines (1.8mm), although it seems fair to include that because we all leave them attached to the tarp.
Also, this was fairly new .74 cuben... the percentage of water weight gain might be even higher with .51 cuben, or with older tarps that absorb more water.
HMG Echo with JB Bivy.jpg