From what I understand the FB app is bloated because it's shipped with all experimental features that are toggled on/off with flags by all the teams that are working on the app. So yeah, that adds up.
And because nobody will uninstall the Facebook app because of that.
I did. Too much space consumption plus notification spam got me to delete it from my device. I'd get rid of Messenger, too, if it weren't so central to my social life.