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The rules in the PRD on this subject seem clear upon a superficial reading, but become somewhat strange when one realizes that the special size modifier applied to combat maneuvers is never stated to replace the typical size modifier applied to all attacks. I'll go into more detail.
When you attempt to perform a combat maneuver, make an attack roll and add your CMB in place of your normal attack bonus. Add any bonuses you currently have on attack rolls due to spells, feats, and other effects. These bonuses must be applicable to the weapon or attack used to perform the maneuver. The DC of this maneuver is your target's Combat Maneuver Defense. Combat maneuvers are attack rolls, so you must roll for concealment and take any other penalties that would normally apply to an attack roll.
Bolding mine for emphasis.
This paragraph fairly clearly spells out that any and all bonuses and penalties associated with a normal attack (using the same weapon) are also levied on combat maneuvers. One such bonus is, of course, the size modifier. Notably, the "special size modifier" table presents bonuses/penalties that are the exact inverse of those in the size modifier table (which is in the Combat -> Armor Class section of the PRD).
Hypothesis: Size category has no impact whatsoever on CMB; the special size category bonus/penalty merely cancels the normal size penalty/bonus.
Evidence for hypothesis: A Dire Wolf's CMB is +8. It has a +4 STR bonus, +3 BAB, and +1 from Weapon Focus (bite). It is Large sized. If the "special size modifier" were applied without the normal size modifier, it would have a +9. That it does not implies that my hypothesis is correct.
Evidence against hypothesis: The Gegenees has a +30 CMB. It has +8 STR modifier, +20 BAB. It is Huge sized. Its highest BAB weapon attacks are made at a +26, reflecting the normal size modifier penalizing its attacks. The CMB, however, is 2 higher than BAB+STR, implying that the special size modifier was not cancelled by the normal size modifier.
Analysis: Monster blocks are inconsistently written/edited for correctly derived combat statistics, and should not be relied upon for answering questions like this. A meaningful answer must be attained from developer commentary, published rules text, or an errata/FAQ.
CMD presents even more weirdness.
A creature's CMD is determined using the following formula:
CMD = 10 + Base attack bonus + Strength modifier + Dexterity modifier + special size modifier
The special size modifier for a creature's Combat Maneuver Defense is as follows: Fine –8, Diminutive –4, Tiny –2, Small –1, Medium +0, Large +1, Huge +2, Gargantuan +4, Colossal +8. Some feats and abilities grant a bonus to your CMD when resisting specific maneuvers. A creature can also add any circumstance, deflection, dodge, insight, luck, morale, profane, and sacred bonuses to AC to its CMD. Any penalties to a creature's AC also apply to its CMD. A flat-footed creature does not add its Dexterity bonus to its CMD.
This, again, seems sensible on first reading, and puzzling upon a careful second and third. The special side modifier is once again the exact inverse of the normal size modifier to AC. Unlike CMB bonuses, all of the bonus types to AC that can also be applied to CMD are specified here. 'Size' is not among these bonuses. This means that a Diminutive creature, for example, really does have a -4 penalty to its CMD. The odd part is that the applicable penalty types received transitively from AC are not specified. This means that a Gargantuan sized creature does not receive a bonus to its CMD, the 'special size modifier' simply cancels out its size penalty to AC.
Hypothesis: Size hurts CMD if you're Small or smaller, and does nothing from Large and up.
I will note that I have never seen a combat run using this counter-intuitive (but I believe textually correct) rules interpretation. RAI is obviously that the size modifier typically applied to AC/attacks are replaced by the 'special' size modifier when evaluating CMB/CMD. However, I have not found this explicitly stated anywhere.
I would appreciate clarification from anyone who can provide rules text from a published source, an FAQ/errata, or a comment from a Paizo developer.
Thank you.
so if you're small your size modifier is 1. For AC and attack rolls you add this number to your stuff. But you take it as a penalty to your CMB and your CMD instead of the bonus.
The opposite is true for large. A -1 for attacks and AC, and +1 to CMB and CMD instead.
so yes, it can be easier to sunder the pixie's armor instead of hitting the pixie.
Dire wolf analysis is incorrect, weapon focus does not raise cmb in general, only for maneuvars using the weapon, so the dire wolf stats support cmb modifier for size replacing size modifier on attack rolls.
The size penalties for AC become a bonus for CMB/CMD.
Reverse engineer any random large or bigger monster in the bestiary.
I have time so I will do it for you.
Huge Earth Elemental
Base Atk +10; CMB +21; CMD 30
BAB= 10
Str mod = 9
Size modifier = 2
Dex Mod-1
CMB=BAB(10)+Str mod(9)+Size mod(2)=21
CMD= BAB(10)+Str mod(9)+ Dex Mod(-1)+Size mod(2)+10=30