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.