The China box pig weight ratings are somewhat deceptive, they are more of a max rating than a recommended rating. Take about 20lbs off of the ratings and you get a better idea of what you can actually cook properly in one.
The boxes are the same length and width, just different depths. The #1 (70lb) is only 8-3/4" deep. The #2 (100lb) is 12-3/4".
Here are some herf China box pig roast photos from a few years ago:
(all photo credits to wyteyes)
The top pig was about a 70lb pig, the lower one was around 85-87lbs IIRC. Notice the difference in the quality/evenness of the cook just by the 15lb difference?
The 100lb #2 box does a fantastic job with 70-80lbs pigs. Papi (the man in the 2nd photo) tried to cook a 96lb pig in the #2 once. It did not work out well. It took 4 hours longer than expected to cook, the leg joints had to be dislocated to fit in the box, and because the pig was so large it had to be flipped during cooking, take a guess what happened to the legs? Yup, they fell off. Try to put a 70lb in a #1 and the loss of the 4" of extra vertical space that the #2 has is going to probably cause you to burn the entire back of the pig in the #1.
70-80lbs feeds about 30 or so, leftovers dependant on how many sides and other food you have for people to eat. At the Cardenas roasts, there is hardly any meat left over by the end of the day.