That seems normal. The copy I have in an m4b ~530 MB (@63kbps). There are various tools like the one you mentioned and (github.com/VarSell/iAmDeaf) which I’m sure does the same thing. Unfortunately I am not too well versed in the actual ripping of content so i dont really know how people get the untouched “highest” bitrate content. But what you did appears to be wihtin the normal range, I would say.
I am not really part of the scene but am part of a community that shares the booty