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?
Best Answer
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: