For the box thing. First we must consider that many different muscles are involved in carrying boxes. And I believe that those who are efficient at carrying boxes are able to divide the load among the most muscles.
If someone does a lot of direct biceps work and significantly strengthens them, make them bigger and improve the capacity to recruit fast-twitch fibers then he will tend to use the biceps a lot more during box carries... the biceps will contract/tense much harder and as a result they will fatigue faster.
One of my former athlete, a very solid football player with a 500lbs+ bench press and who was over 300lbs of solid mass had twin babies... he could hold them maybe 5 minutes in his arms and has to stop because his arms fatigue... but his wife can hold them forever despite being less than 1/2 his size and 1/5th of his strength.
That having seen said, if biceps mass is build while doing a lot of loaded carries, there will be a better transfer.