Thrown weapons can be thrown up to 5 increments. So a spear with a 20 foot range increment can be thrown up to 100 feet.
And as far as I know there is nothing that changes range based on size for either thrown weapons or projectile weapons. IT stays the same regardless of size.
It starts at your character
If you have to have a logical justification, then remember that larger creatures get penalties to their attack due to their size, and they tend to have terrible dexterity (almost every polymorph spell and such that increases size reduces dexterity, and things like dragon can often have single digit touch AC).
So you aren't exactly the most graceful or precise creature with ranged attacks when you are that big. You might get a decent dex score, but that is 'in spite of' your size.
So even with 10ft natural reach, I can only accurately throw a spear 20ft? Or an improvised weapon I could only throw it as far as I could reach accurately? Or does the range of the thrown weapon start at the end of my natural reach?
It starts at the edge of your space, not the edge of your reach.
And it gets worse. Some creatures have a reach long enough that they can strike a creature with a melee attack, but take a range penalty when throwing something at the same target. (Titans have a 30' reach, so can strike a target 30 feet away with a melee attack. Attacking the same creature with a thrown weapon would give a penalty for range. With a reach weapon, he can hit someone 60 feet away as a melee attack, bt can only throw a dagger at someone 50 feet away, and with a -8 penalty for range.)
No, it does not make sense. It isn't the only thing that doesn't make sense (A 2' Small sized creature with a dagger can poke someone at the same distance as an 8 foot Medium sized monster with a regular spear. Both have the same 5' reach.) Range should scale with size and strength. But the rules were kept simple.
Basically- the rules are mostly made for medium sized characters, and it breaks down when you go beyond that.
And most of the really big enemies already have their own shticks that consist of melee attacks, magic, or weird abilities that don't use ranged increments like fire breath.
Again- you can mentally justify why their melee attacks work, since they just use their large size to sweep the area. But a ranged attack would need more precision than that.
As a justification with no attempt at applying actual math to it, assume that larger thrown weapons have an increased weight and drag sufficient to make them no better than their medium sized counterparts.
As a house rule, suggest allowing creature to throw items an additional number of ranged increments equal to their strength modifier.