Piping for this bathroom is measured by inside diameter, not outside but we are pretty sure of the sizes you're dealing with.
Take pictures if you can; this will help you help us answer your questions.
The bigger pipe mentioned is the one for the toilet; rough from center should be 11-13" off the rough-in wall. AS Cadet/Kohler/Toto's are notorious now for being off the wall on a 12" rough.
One of those pipes directly behind the toilet probably serves as the vent....the other the vent for the tub or catching the lav.