When you read a particularly shocking account, one possibility is that the experience was so overpowering and the actual knowledge so limited that the details are exaggerated. That's especially true if a shocking story advances one's own interest. The other possibility, of course, is that the details are true and what happened was just that shocking.
The question is just how much cannibalism there was.
From Diaz’ account, the practice was widespread, but the numbers did not seem enormous. Perhaps a few people in each major town every couple of months or so.
It may have been much, much higher in Mexico City because of the high numbers of human sacrifices. A source on the net says estimated numbers that the Aztecs sacrificed vary from 20,000 to 250,000 people per year. That is a lot of meat, but there were about 5 million people in the Aztec Empire.
Diaz indicates that the arms and legs were taken for food, but not the head or torso. Hard to know exactly when you do not speak the language and are at war.
100,000 would easily be within the capacity of a 5 million population. It would be 1 of 50 people a year.
From animal analogies, one person would produce about 50 lbs of meat, or about 1 pound of meat per person per year.