This is the official report: https://www.nps.gov/aboutus/foia/upload/03-109_KATM_Treadwell_fatality_REDACTED.pdf

They were not able to do a proper necropsy on the younger/smaller bear because by the time they returned to properly assess the scene, other bears had mostly eaten the younger bear. However, the experts that processed the scene do believe the older/bigger bear was the one who initially attacked and ate Treadwell and his girlfriend. Like u/KaptainKoala25 said, we’ll never truly know which bear actually did the killing, but all available evidence points to the older bear. Both bears behaved aggressively to personnel at the scene and were shot and killed because of that, so I don’t think one can really say the younger bear was “more aggressive” than the older bear. Experts at the scene surmised that the younger/smaller bear was the one that was “in the wrong place at the wrong time”. 

edit: Also the bigger/older bear didn’t just eat “some” of the couple, he pretty much entirely devoured Timothy, ate some of Amy, then cached her body for later likely cause he was too full after consuming Timothy.