In regards to why Superman is struggling with the plane, but can lift something extraordinarily larger than that with the same amount of strength, well here's the deal: Superman holds back his strength 24/7, almost enough to the point where he forgets about the majority of his reserve of strength unless in moments of absolute extreme where it means victory or death. Now, the plane sequence, I assume what's really going on, is the writers just want to show the maximum amount of drama possible, by giving the audience a "oh no, Superman might not have what it takes", in order to have maximum effect, this is what was needed instead of having him fly infront of the plane and stop it with his pinky as if it were a cartoon. I'm one of the Superman fans that believes this character should be able to move a mountain with a grunt and a vein bulging on his face, but I also realise that for the whole American/World audience, they need to make scenes like this, more dramatic in the way they perfectly did. Now with the whole lifting you know what into outer space, this will most likely be one of those scenes where he is A: either Super Charged by the sun, or B: Completely pissed and exerting somewhat his full strength potential in order to get the job done.
PS: Thx for the vid links fellas.