That would be true if RSA scaled proportionally with the number of bits, but the exponent involved is much lower than 1. 1024->2048 gives you around the same difficulty as adding 30 bits to a symmetric key.
I stand corrected, thanks! 2^30 still means a billion times better.
It's also only true so long as we don't discover more efficient ways of factoring large numbers. We haven't come up with any dramatic improvements lately, but it's always possible that something will come up. Symmetric crypto systems like AES are on much firmer ground, as they don't depend as heavily on the difficulty of any single mathematical problem.