Actually, if the Deathwatch RPG is anything to go by, carrying another space marine (just him and his armour essentially, though weapons and the like don't add that much more weight) puts an average-strength Astartes wearing power armour at just under half of light load... which means it does nothing at all to encumber him other than taking up an arm.
For example, my techmarine in a game could easily drag a Rhino around if he needed to, as it was within his drag load. he'd be moving at half speed, but that's still moving at six meters per turn. The assault marine in our group (who was stronger even than my techmarine, discounting the servo-arm) could lift above his head that same Rhino.
Actually, a lot of that was remembered off the top of my head from a game we had going a few months ago, so it could have been a Chimera and not a Rhino that we were comparing against... but the point still stands that Astartes, especially with their armour, are friggin' strong.