Big fish produce a larger number of eggs, and a larger number of offspring. Their fish eggs are also more VIABLE (greater chance to survive) than a smaller fishes eggs.
Size is heritable. Larger fish have genetic traits that make them large and they pass these traits on to their offspring. This would include an increased speed to maturation, size and the potential for heightened survival instincts.
Larger, older fish know the best places to lay their eggs and depending on the species, how to best protect them.
We really like the article from Outdoor Canada - Why You Should ALWAYS, ALWAYS, ALWAYS release big fish: