Can you imagine the amount of programming and memory needed for millions of miles of nothing. It's 93 million miles to the sun and 390 million to Jupiter and if you get lost you are going to be seriously bored. I would like the odd space battle/mission/instance but sitting there for a few months getting to Mars ain't my bag.
Naturally I'm not saying make the whole lot accurate to scale, but Infinity: The Quest For Earth is doing more than that, with the ability to have PVP and to establish cities on other planets. If a PC game, which the developer says can run on a relatively mid-end PC smoothly, can do it, a next-gen game can too. Of course, I have no idea if the current-gen Xbox or PS3 would be able to try it.