Carry That Weight Lyrics Meaning

The lyrics are McCartney's. When you consider the tense legal disputes happening between McCartney and Lennon (along with the other two Beatles on Lennon's side) around that time, I think it's likely that it is McCartney's warning that the ramifications of their decision to hire Klein as their manager would be felt for a long time. The "boy" at the start of the first line could be addressing Lennon personally, or (more likely, I think) is just an exasperated exclamation, like "Jeez!" It's McCartney saying "My God! You're gonna be paying a heavy price for a long time."

Another possibility that has occurred to me is that the "weight" is Yoko Ono or, more broadly, Lennon's decision to embrace a new life with Ono at the expense of his marriage and his relationship with his son, Julian. McCartney had very strong sympathies with Cynthia and Julian at the time of the breakup, could see Ono's influence and constant presence becoming ever more powerful, and was not exactly Ono's biggest fan, although diplomacy would have dictated that his feelings could not be expressed other than indirectly through a lyric.