Comparison among algorithm based on key length
Usually we only consider those keys, we can actually use. There is no restriction about the bits in a symmetric key, you can use all of them. For asymmetric encryption schemes, both keys need to fulfill some constraints to actually work. For example:

In RSA, $e$ has to be chosen coprime to $\phi(n)$, and you need $ed=1 $ mod $\phi(n)$0. What does it mean? Yes, there are certain values of $e$ (and $d$) which do not work.

edit: Even numbers never work, because $\phi(n)= (p-1)(q-1)$ is always divisible by 4. Other than that, the fastest way is to pick a random odd number for $e$, and check if it is coprime to $\phi(n)$.

What the reason is, that we can not blindly compare bit sizes of keys, is that attacks on symmetric and asymmetric encryption schemes are fundamentally different. It is a bit like comparing apples and oranges. So a 200 bit symmetric key is just something different than a 200 bit asymmetric key.

However, there are heuristics (not proven) to compare the strengths of these keys. For example NIST provides information about that here.

A very good read is Universal security; from bits and mips to pools, lakes -- and beyond, which tries to accommodate the the irritating scale of bits in a humorous way.