All attackers—not just the pinning creature—treat a pinned creature's Dexterity as 0 for determining the pinned creature's Armor Class
The Player's Handbook's glossary describes pinned as "held immobile (but not helpless) in a grapple." That immobility is represented by treating the creature's Dexterity as 0 for determining the creature's Armor Class.1
Thus, initially, the grappling creature and the grappled creature lose their Dexterity bonuses to Armor Class against attacks made by creatures not involved in their grapple (PH 156).
Then the pinned creature—who's also grappled as that effect has yet to be remedied—is treated as having a Dexterity score of 0 for determining Armor Class (PH 151), but the pinned creature is not paralyzed (as a creature with an actual Dexterity score of 0 usually is) (PH 304, 311), and the pinned creature is not helpless (PH 153).
Further, the pinned creature suffers an additional −4 penalty to Armor Class against attacks made by creatures not involved in his grapple (PH 156).2
Example
Arboc (base attack bonus +6 and usually Dexterity 13 and AC 20) and Baracs (usually Dexterity 16 and AC 17) are already grappling, making their Armor Classes, respectively, 19 and 14 against attacks launched by those not involved in the grapple.
Arboc, on his turn, wins an opposed grapple check at his full base attack bonus against Baracs. Arboc picks to pin Baracs. Baracs, now virtually immobile has his Dexterity for the purposes of determining his Armor Class reduced to 0, therefore an armor class of 9. Further, Baracs suffers a −4 penalty to Armor Class against attacks launched by those not involved in the grapple, making his Armor Class against those foes 5.
If this seems excessive, try imagining being pinned as failing a second saving throw against an effect. Since failing the first could've killed the creature, probably either grappling is the creature's specialty, the grappling creature wants the grappled creature alive, or nobody's casting spells.
Grappling is one of the primary reasons many high-level creatures have access to some kind of freedom of movement effect (e.g. the ring of freedom of movement (DMG 232) (40,000 gp; 0 lbs.)).
- The Rules Compendium defines immobilized as "An immobilized creature can’t move out of the space it was in when it became immobilized. It otherwise functions normally unless it’s flying. Immobilized flying creatures that have the ability to hover can maintain their initial altitude. All other flying creatures subjected to this condition descend at a rate of 20 feet per round until they reach the ground, taking no falling damage" (35). The RC doesn't link that condition to the pinned condition, however, stating separately that a pinned creature "can’t move, so its Dexterity is considered to be 0 for the purpose of determining AC (−5 modifier)" (35).
- I speculate this represents the pinning creature's ability to steer the pinned creature into oncoming attacks. D&D 3.X, so far as I'm aware, omits a grapple benefit that permits actively employing the pinned creature as a shield or as cover.
As RAW, no, it does not
Being considered a size higher for carrying capacity and push, drag, or lift force is not the same as being a size higher for all purposes, including grappling.
You count as one size larger when determining your carrying capacity
and the weight you can push, drag, or lift.
As RAI, maybe
The loop hole is that the PHB has no reference on why and how the grapple-er can move without speed penalty if it is two size higher, and the DMG does not touch the subject. There are two potential scenarios for why a grapple-er one size or lower has it speed reduced, though; the grappled creature have enough leverage to make the movement difficult or that the grapple-er cannot carry comfortable enough the grappled creature as in "difficult terrain", and that the weight of the creature is the deterrent.
In the first case, since Powerful Build does not increase per-se the size, it is safe to assume that it does not help to increase the comfort or reduce the leverage and, such, the speed is halved.
In the second case, where is the weight that maters, things change. Powerful Build improves the carrying, dragging, and lift capacity, therefore the grapple-er creature does not have its speed halved.
Which one it is
The only evidence I found is in favor of is that weight is the reason for the reduction in speed. The first part is in the text of moving a grappled creature (emphasis mine).
When you move, you can drag or carry the grappled creature with you,
but your speed is halved, unless the creature is two or more sizes
smaller than you.
As far as I can tell, there are no rules on how you drag or, particularly, carry a grappled creature. You can carry over you head a grapple creature as if it were a sack of potatoes. In PHB 176 there is a segment that shows how can you drag, carry and lift something in particular and, as shown, it is a STR and weight contest in which Powerful Build should work as intended.
Push, Drag, or Lift. You can push, drag, or lift a weight in pounds up
to twice your carrying capacity (or 30 times your Strength score).
Bottom line
At the end is the DM that has the last word. Talk to your DM and present the evidence. If you are the DM decide what it is best for your campaign. If a particular decision is more powerful that you expected it is in your right to take back the decision.
Best Answer
It's reduced to a quarter of your regular speed
So when you want to start dragging one creature, that will halve your speed. If your speed was 40 its now 20. In addition to dragging the first creature you want to drag a second creature, which again applies the halving, so your current speed of 20 is now 10.