The Trex is smaller in cross section which is true but that is because it is better suited for curved decks and fancy layouts. When I was shopping I had the Trex and the Portico decking to choose from. The lumber yard owner (friend) asked me what I was doing for a deck. I told him just a standard raised 12x30, nothing special. He recommended the Portico saying the Trex was much more pliable and better suited for curves like this:
He took a forklift and picked up a pallet of Trex 16ft'ers and lifted it from the center, the ends of the boards touched the ground from almost 4ft up.

The he picked up the same sized pallet of Portico 16ft'ers and lifted them up and they barely flexed more than a foot or so. He said they are stronger but don't try making a curve with them. :laughing:
Each has their place but reliability and durability are very both much higher than PT.