The reason that I'm going with on superman's strength in man of steel is what the jorel A.I. Told Clark. he didn't know what clark's limit was, so the only way to find out is to keep pushing until you peak out.
I had a problem with superman struggling with the oil rig until i hears jorel's remark.
He struggled because that was the heaviest thing he held up to that point. So by doing that his strength increased.
Hmm... He's a little bit like a saiyin in that sense.