Encumbrance in low G

[Phantasm]

In this case, where a person can bench-press an automobile without too much effort due to low gravity, you're well within your rights as GM to rule that encumbrance due to weight is applicable to "long-distance" feats like hiking, while encumbrance due to mass is applicable to "immediate" feats like jumping and ducking in combat.

Another take would be to apply any positive SM of whatever is being carried as a penalty to the feat. For example, your guy may be able to carry that SM +3 half ton compact automobile on his back without killing his muscles, but he's still carrying an SM +3 compact automobile. That makes him a much larger object overall.