When I bought it (or rather when I dragged my dad to the GAME store to buy it for me for my birthday because he is clueless about video games), the man behind the counter said that the average play time to complete the basic game was around 24 hours, and since I spend 40 hours plus on other FF installments, this is a dramatic shortening, so yes it is short in comparison, but I think it was meant to be like that.

The sidequests, e.g. getting the Celestial Weapons, Blitzball, fiend hunting, Dark Aeons (Europe), Penance (also only Europe, I think) etc. take more time to master, add more depth to the story, and is where the extensions of the game lies, as devil's phoenix said.

I also agree with Studder; FFX-2 definitely doesn't count!