I have seen zero evidence that Reeve had a lot of muscle Pre-SM1. If he did, then yes, it would be easier to re-gain 30 pounds than to gain it in the first place.
He went from 175 or so, to 215, under the supervision of a competitive bodybuilder at a time when AAS was not even considered bad for you.
Being taller does not make it much easier to gain muscle.
Harder to fill out in fact, because they gain it at the same rate a shorter guy does, or at best not much faster, but since they need more to fill out, it actually takes longer.
That is also the reason there are not a lot of tall bodybuilders. Over 6ft is only small%, with about 90%+ of them being 5'7" to 5'11".
Not that we need a bodybuilder to play Superman however. My point only being that for a guy nearing 40, who has not previously had a very impressive build, there are more things that go into getting into shape. Considering most men have only 50% the testosterone level at 40 they had at the age of 18, there need to be some extras in the medical department...
You know, like the natural supplements Stallone got in a fiasco with Astralian customs over.
You are STILL looking at it from the modern paradigm.
In the 1980's it was not yet considered cheating.
The paradigm then was you were a fool not to.
edit...
Even today, if an actor like Stallone has a perscription for testosterone and hGH, its not exactly like he is playing on a sports field.
Layne Norton is one of the best natural bodybuilders in the world. Ask him if he thinks a man over the age of 40, who has never been that muscular, is going to gain 30 pounds of muscle and strip 15 pounds of bodyfat in a few months.