The actual length itself is not the issue belying many installments in the series... length is contingent upon the plot and its resolution. Rather, the issue is that there is a lack of mytharc fulfillment in the games. Indeed, whilst many of the games are based around grand archetypes of the Hero and his journey, often involving a descent and then resurfacing after having slain a mythological "dragon", in Zelda this part is often cut short and not played out in a properly fulfilling manner. The Hero's journey is the bulk of the game, involving a departure (that is often traumatic) and entering a hostile world, on a quest to defeat a grand evil and restore a [cosmic] balance.
Longer games would likely be a byproduct, certainly, but of most import is a storyline in which not only is the full mythic drama expanded upon, but a zodiacal conclusion is put forth, replete with a celebration of the return of the dead godhead/Hero, who now having completed his cycle and restored the balance can live out the promise of Spring.