DMG 222 is pretty clear that you use one carrying/lifting/dragging calculation for all PCs. Am I missing an exception for Tiny creatures, or are Pixies just that incredibly strong?
There are no size modifiers to character loads in 4e. This is completely represented by a creature's strength score. A pixie with a Str 10 can carry 100lbs at normal load.
That's not to say that a pixie should be allowed to carry normal sized items on their person. In this case I'd allow a pixie to use shrink as an at-will out of combat and shrink whatever items he must carry down to size to fit into his pack.
The idea to me is that a pixie with str 10 can't actually carry 100lbs, but they can carry items equivalent to 100lbs for medium/small creatures shrunken down to pixie size. However, this is completely a role-play decision as there is no mechanical issue with the pixie carrying a straight up 100lbs of stuff.
The question arose if a Pixie could carry a 150lb human. Mechanically, if the Pixie's carry weight is over 150lbs, the answer is a flat yes. As a DM I think you'd be justified ruling the other way, but following the rules as written indicate yes.
For the most part 4e has completely done away with size modifiers and restrictions (the only restriction is on weapon size, and the only real benefit is the space sharing thing). Favoring abstraction over simulation.
wax eagle's answer is correct if you want the rulebook answer.
However as a Dungeon master, I house-ruled some mechanics for it:
I compared the weight of the pixie to the weight of a human, then used those proportions to scale down the Pixie's carry load. It meant that pixies weren't able to carry much more than their own weight unless they shrunk these item to size. If an item was shrunk to size, I would use the same proportion to calculate it's new weight.
This helped remove illogical situations like a pixie carrying it's fallen Human comrade on its shoulder, or flying with huge treasure chests a hundred times its size. One of my players even boosted his strength high enough that he would have been able to carry a Dragonborn on a swing. It doesn't take too much thought to see why this would be illogical. The only reason I can think to keep the original rules for this would be if you wanted to have a sort of Ant-man aspect to the pixie; allowing you to pack a huge punch in a tiny space. And even then, all you need to do is boost the pixies weight to 120-180 and the house-rule would scale the carry-load.