The zone would be twice as big if it took 10 minutes. In theory the bigger the zone the more content and stuff to do, and likewise the smaller a zone the less is there to do. At the very least, a bigger zone allows for more variety both in environment and creatures. Sure, you can put 20 dailies in a small zone, but it's gonna be vs. the same 4 or 5 creatures. Whereas a bigger zone encourages more variety.
Personally I feel Blizzard deliberately chose to make a smaller zone because they wanted to avoid people complaining about the lack of flying in the new zone. The zone is so small that even without flying it's quick to traverse.