To be accurate, this isn't true. There are no official Large PC races, so you can't comment on the "main benefit" because there are none. If you read the posts above, however, you'll see other options that I think support what I said. Namely, using "next step" dice. That's an example, and I'm sure there are others that would contribute solely to vertical optimization. PCs would surely want threatening reach, wouldn't they? +4 to strength instead of only +2?
PCs would surely want +4 strength, but are unlikely to get it

I think the goliath is a good example. It gets some large like benefits. Those could probably be pushed a bit, with some added penalties (or less bonuses in other areas).
My point is there is no blanket list of big bonuses one receives when they are large, like you get in 3e. Because it hasn't been defined, someone taking on the challenge has a lot more ease in designing balanced mechanics.
In 3e this is much more difficult, because being large gives you such a "large" list of bonuses that the balance point is already off.