It's true, but that's a valid way to approach the question. The problem is that grandparent will have more misses (people who legitimately believe that the earth is larger than the sun by the assumed metric) than hits (people who think outside of the box by using a different metric).

Actually, a lot of people use the "what's more directly relevant to me" metric. Thus, you have a lot of people who pick "earth" as an answer even though they are not delusional. At least not in this specific way.

If you were looking at a painting it would sometimes not be stupid at all to say "the house is bigger than the sun"

It's a dating site. The point is to have fun and meet like-minded people.

Well, what is more likely: That people are answering the question that way because they are wise-cracking or just because they are stupid?

As such, for people that don't like geeks who interpret everything literally, answering "the sun" might be a good strategy, as I doubt anyone else would make this question mandatory.