Never heard of Hero engine?
Also yes, Cryengine is good at a single level, it's not meant (at least originally) for a streamable content like GTA, or Skyrim etc.
I'm sure they could patch in that code, but there's only so much lipstick you can put on a pig.
That being said, I think that when you get rid of all the palm trees, hundreds of AI soldiers, and shit tons of detail that Crysis had, and just put it into outer space, which is essentially just some objects floating in a skybox, then the real scale of how big you can make a "single player" map would get truly gargantuan, like planet-sized.
Other than the destructible trees, nice water, and well-done AI of Crysis, the thing that stood out the most was the scale of the levels. I'd imagine that there's still more geometry and detail in the first level of Crysis than probably a whole solar system in SC. Space is pretty barren, no vegetation, and shit is really far apart, easy to optimize for (aka stop rendering shit you can't see, ie. going indoors the engine stops rendering outdoors if you can't see it)