I think the level of exploration in CD's games compared to Core Design's comes down to the way the games were built. In Legend and beyond, interactable objects were placed onto the scenery so Lara had means to make her way around. In the original Tomb Raiders, interactable objects were the scenery. Lara was designed to be able to climb, run and walk on blocks that were then used to the make up the levels. Slap any texture on them, but Lara could still hoist herself onto it if it was low enough. So, while the original game is technologically insuperior to Legend, it still manages to win in the explorability stakes. If you can see the top of something in Tomb Raider, chances are you'll be able to get on to it (whether it will be useful or not). And there was always that chance the Core would have left something a click too low and you'd be able to explore things they never even wanted you to see.
Another technological constraint in Tomb Raider is the draw distance, which means that the game cannot draw things after a certain distance away from Lara. For example, when she ventures into a large room, whatever is on the other end of it is slowly unveiled the closer Lara gets. Not very realistic (and it's a rare affliction in new games), but this little blip can create a whole lot of atmospehere. Take, for example, my recent excursion in virtual Peru:
All in all, I don't think improved games technology wins everything. There are some things in Tomb Raider that work better because of flaws. St. Francis Folly wouldn't be the same if the draw distance could stretch to the floor.