I think there are some hidden attributes that are added by MM4 because the same file is larger when saved from MM4 to the library than if saved by Poser. But whatever the additional size is, is seems to have no effect on how long the file takes to load. To lean it up, just load the file and save it again in Poser with the same name.

If this is a character that is set = the shape morph settings are what you want to keep - you can make this character available from a much smaller file - ~5meg. and almost all of this due to head expression morphs.

There are two ways to do this:
completely character specific - a cr2 for every character
generic - a basic cr2 that becomes a character with a special sort of pose file.