Yes, the S is large. The 3 isn't as small as you think, but it's not as large as the S of course. Maybe you should wait until you sit in one to judge. As far as speed, the base 3 is slower, but performance models will be much faster, so you're assessment is inaccurate. I don't even know what "twice as slow" even means. That's not true in any math equation. It would be more akin to a 5 series vs 7 series BMW, only if the 5 series never existed so people were stuck buying a 7 series for years until a 5 series was finally released. See how that works?
It is fair to compare them since they are from the same manufacturer, and only good EV's on the market. Just because you say you can't compare them doesn't make it so. A lot of people are going to have to make this decision in the near future.
I think a lot of people thought the model 3 would be an inferior vehicle compared to the S. Maybe it will be, but from what has shown it has blown people away. I certainly wasn't expecting it to be as good as it sounds. You say the S is 80,000 premium vehicle, but Teslas margins are a lot higher, and it was more expensive to produce. If Tesla re-did the S, and made it a mass market car, surely they could bring the price down a lot. That's essentially what the 3 is. As far as I can tell, it stays true to the S at a much better price point.
Stop telling people what to compare, every situation is different, every persons needs are different. When the 3 is released, these models will not be as different as you think.