Size bonus from enlarge person when naturally large?

Question

I made a character using the race builder and spent the points required to be large. During a comment conversation it was pointed out to me that I might be doing something wrong here (besides that you shouldn't be having conversations in comments).

Large (7 RP): Prerequisite: Humanoids taking this quality must have the giant subtype; Benefit: Large creatures gain a +2 size bonus to Strength and a –2 size penalty to Dexterity. Large races take a –1 size penalty to their AC, a –1 size penalty on attack rolls, a +1 bonus on combat maneuver checks and to their CMD, and a –4 size penalty on Stealth checks. A Large creature takes up a space that is 10 feet by 10 feet and has a reach of 5 feet.

Since enlarge person has similar wording

The target gains a +2 size bonus to Strength, a -2 size penalty to Dexterity (to a minimum of 1), and a -1 penalty on attack rolls and AC due to its increased size.

Then there is mythic enlarge

If the target grows two sizes, its weight increases by a factor of 25, it gains a +4 size bonus to Strength, and takes a –4 size penalty to Dexterity and a –2 penalty on attack rolls and to AC because of its increased size.

Now mythic seems to suggest that there is a relation between size and having a bonus to str and penality to dex, but looking at stuff I cant actually find this as a fact. So does that mean the +2 size bonus I have from naturally being large does not become +4 when I become huge, it simply stays as +2? What about the dex, does it stay at -2 or become -4?

Answer 1

The +4 to Strength in mythic enlarge person is a benefit from that spell, not a general rule. Size bonuses and penalties don't stack - so your Large character (specifically because you're a race-builder Large character, and so have built-in size bonuses) neither gains Strength nor loses Dexterity from being affected by enlarge person, you simply go up a size category (and get the normal effects of changing a size category - your equipment weighs more, you weigh more, you can carry more, your size modifier to attack rolls and CMB increases, your attacks roll larger dice). If you were subjected to mythic enlarge person, you'd gain +2 Str and -2 Dex for the duration, as the +4/-4 would apply over your natural +2/-2.

See also: This question about size modifiers to Strength and Dexterity

Special thanks to DM_Blake of the Paizo forums, because I found this quote much faster there, regarding stacking penalties:

Pathfinder Core Rules, Chapter 9, Magic, page 208 (depending on printing) wrote:

The Spell's Result

Bonus Types

Usually, a bonus has a type that indicates how the spell grants the bonus. The important aspect of bonus types is that two bonuses of the same type don't generally stack. With the exception of dodge bonuses, most circumstance bonuses, and racial bonuses, only the better bonus of a given type works (see Combining Magical Effects). The same principle applies to penalties — a character taking two or more penalties of the same type applies only the worst one, although most penalties have no type and thus always stack. Bonuses without a type always stack, unless they are from the same source.

Answer 2

The glossary entry (from Table: Bonus Types and Effects) for size bonus states:

A size bonus or penalty is derived from a creature's size category.

Going by this definition of a size bonus the simple fact that you are changing size means that your AC, ability scores, and all the other things that size affects will change. The spells, although not worded particularly well under this interpretation, can be understood to clarify what the change in size does (i.e. the spells change your size. The change in size changes your stats, and that effect is explained in the rules text of the spell), not that the spells themselves bestow the bonus. Thus the rule about combining spell bonuses is circumvented because in fact when your size changes you lose your old bonus and gain a new, larger bonus.

Finally, I believe it makes no sense for a larger creature to be just as strong as it was before just because "the bonuses don't stack".