I'm not a plumber. Just trying to learn.
What vents the toilet? If it's wet vented by the lav does that vent need to be 2"?
Toilet venting is different than for other things. For most things, the venting prevents siphoning in the trap. In a toilet, the trap is part of the toilet. The trap siphons out on purpose. The trap then gets refilled via the refill tube and the overflow tube as the tank refills.
The venting then keeps the slug of water and solids from being held back. In UPC, the toilet is last in the wet-venting line. In IPC, that is not required. There must be a roof vent in the system to prevent a pressurized system, but the lavatory and the rest of the group could be vented with an AAV.
UPC wants the wet vent for the toilet to be 2 inches (or more). IPC, not sure. 1.5 inches after a lavatory may be sufficient, but I am not sure of that.