50 Cent is definitely taller and heavier than Conor sure, but Conor's reach, like many strikers, is unusually long for a man his height. He's 5'9" with a 6'2" wingspan. 50 Cent doesn't appear to have particularly long arms, so their reaches are probably comparable, with 50's likely falling within three inches of his 6'0" height.
So given that there likely is no big difference in reach, Conor's relatively shorter height could also play into his favor, since punching up generally carries more force than down, due to being able to throw more of your body into the blow. This would help alleviate any difference in punching power their respective sizes might grant them.
Plus, you know, 50 Cent hasn't regularly trained in even boxing (boxing is poopybutthole against more complete fighters) since he was a ****ing teenager and is a forty year old man, while Conor is young and is a world-class mixed martial artist. Of course Conor would win. 50 Cent is bigger than Conor, but he's not Haffthor Bjornnson or the Big Show levels of big, where Conor would be completely overpowered and unable to check their chins. I've seen guys significantly bigger and more powerful than 50 get beaten by fighters about Conor's size.
50 is struggling so hard to stay relevant. I wonder if he'll release a new album filled with Conor disses? Dude needs to let it go. He's fallen off, and probably won't ever recapture his glory days.