I think that since most people know Secretariats story, the movie makers are trying to play up Penny's role, thus making it new. I am going to guess it takes the focus off the horse, which is not something that should be done to a superstar like Secretariat.
And it is accurate that Big Red's heart was larger than normal.... google Secretariat'e heart...... Normal horse heart= 8.5lbs. Secretariat's heart= approximately 22lbs. Sham's heart= 18lbs. Apparently a big heart is something passed down the female side of genetics pool so of course his get would not have the same large organ....
As for who would win...... I am thinking that Secretariat would- he would let Seabiscuit play with him a bit until the far turn, and then, as always, he would take off :)
"Equine-facilitated therapy employs a form of biofeedback for practicing self-awareness, emotional management, and relationship skills that human role-playing exercises and discussion groups cannot begin to access." Linda Kohanov (The Tao of Equus)