According to the wiki,
This is not true. The off-hand being light or not has nothing to do with it, it's the size that matters (insert The Office reference here).If you wield a second weapon in your off-hand, you can get one extra attack per round with that weapon. When fighting in this way you suffer a –6 penalty with your regular attack or attacks with your primary hand and a –10 penalty to the attack with your off hand. If your off-hand weapon is light the penalties are reduced by 2 each.
If your off-hand weapon is one or more size categories lower than your character, then you get the penalty reduced by 2. So for example, a human (medium size) gets the penalty reduced if the off-hand is a small or tiny weapon, like a short sword or dagger. But if the off-hand is a rapier ( a medium size light weapon), the penalty is NOT reduced.
This distinction becomes even more important for small characters (halflings and gnomes) because if they have a small weapon in the off-hand they do NOT get the penalty reduced even if the off-hand weapon is light. Their off-hand weapon must be size category TINY (daggers and kukri) to get the reduced penalty, an off-hand short sword does NOT get the reduced penalty for halflings and gnomes.
Note that the size comparison is ONLY between the size category of the character and the size category of the off-hand weapon, NOT between the two weapons. A medium size character can wield two short swords, or a rapier and a short sword, or a greatsword (with Monkey Grip) and a dagger, whatever, as long as the off-hand weapon is small or tiny. This means that halflings and gnomes cannot wield two short swords without the harsher penalties, their off-hand weapon MUST be a dagger or kukri because those are the only two tiny weapons in the game. (idk why the light hammer isn't tiny).
Theoretically, this also means a giant NPC could have a greatsword in the main hand and a longsword in the off-hand and benefit from the reduced penalties, but I can only test with PCs.
