What you read on "some random forums" was probably referring to generalised and/or automatic serialisation. Such tools may not designed for space optimisation. For example, they may not allow you to express domain specific knowledge such as the range of a "health" variable, etc.
All the serialization libraries I know of eliminate padding that compilers have added. They also allow marshalling of single bytes. They may not support bit level access though. I'm not aware of a library that supports that.