Let's assume that the large creature with reach weapon (reach 20') does not have a feat to not provoke AoO.
And a medium humanoid with a longsword (reach 5') is standing out of his own reach but in reach for the Large one.
The large creature goes for a sunder attack.
You can use a melee attack with a slashing or bludgeoning weapon to strike a weapon or shield that your opponent is holding. If you’re attempting to sunder a weapon or shield, follow the steps outlined here. (Attacking held objects other than weapons or shields is covered below.)
Step 1: Attack of Opportunity. You provoke an attack of opportunity from the target whose weapon or shield you are trying to sunder. (If you have the Improved Sunder feat, you don’t incur an attack of opportunity for making the attempt.)
Should that be only if the medium one can reach him?
or given by RAW, screw physics?
An extra question...
is in pathfinder (besides the CMB CMD stuff) anyway different?