1AU=93 Million Miles
Jupiter is over 450 million miles away. Comets start to significantly outgas just inside Jupiters Orbit. 30 million and 100 million miles are well into the warm part of the INNER solar system.
Comets are pretty large compared to most NEA's. Their comas are very diffuse. They only loosse a very small fraction of their mass on each pass through the inner solar system. A break up probably requires a very violent outgas as well as a relatively weak makeup.
We've certainly learned that comets are not a uniform class of objects.
Regards,
Mark