Well, the MC-80s hold 120 fighters, including the B-wing, and is over twice the size of an ISD, based on the models, so the 70 seems about right.
PS6 is not rubbish, but canon indicates that Shara was an incredible pilot, not just a good one. I mean, she became a triple ace over the course of a single battle according to "Before the Awakening", and in "Shattered Empire" she held off an entire Star Destroyer's worth of TIEs in an antique N-1 while Princess Leia and Queen Soruna were busy shooting down the Imperial satellites.
Is that when a star Destroyer's worth of TIEs is just over seventy? Seriously the h*, what's that about?
Edit: Wrong numbers.
Based on the models from the films you mean? The models in Armada make the MC-80 miles smaller than an ISD. Still, I'd expect it to be hundreds, wouldn't you?