Down with a word, in this case, mighty, as non-germane in a discussion about writing? I'd rather open things up, really, than close them down.
The quote was of its time. Of course it's old-fashioned. Dated, absolutely, and nowadays we'd probably choose other words, epic or monumental or classic, but I don't see why it is limited in suggestion to themes of masculine proportions of strength. We may think of a mighty warrior, but a mighty warrior could be Brunnhilde or Hippolyta. Or we may think of a mighty orator, poet, artist, stone, mountain, river....ocean...storm....or a mighty problem or puzzle or conundrum.
I think of a digital publication called
The Mighty, which talks about living with disability. 'Mighty' is not quite yet defunct.
@Hannah F observes, even a very slender book can be an epic. Totally. The Little Prince or Jonathan Livingstone Seagull come to mind.
Dorothy Dunnett's King Hereafter is a doorstop of a book. I'm in awe of the writer's sheer physical achievement, and wonder how, at what point in her thinking, what was the driver, the trigger, such that she decided to embark on such an immense fiction project?
This is a modern epic, or what Melville might have called mighty. portrayal of MacBeth as the man the history books suggest he really might have been, once we get away from Shakespeare, not forgetting who was his patron. But it is every bit as much the story of Lady MacBeth too, Gruoch. Themes; the power of land, the nature of power, the cost of power, the nature and power of love- and hate. Lady MacBeth tries to kill him, and he says, can it be possible you do not know I love you?
Romanticism was a movement in two disguises. There was the green and rolling Picturesque, and there was the craggy, looming Sublime, specifically characterized as androgynous, rather than inherently masculine. I guess Melville meant by 'mighty' to suggest the Sublime, and it is the sheer scope and scale of the vision that makes it epic- mighty. , the breadth of scene, the complexity of events, the depth of research but above all the emotional intensity and depth, as with Ahab's fateful obsession for the whale - and the whale's long memory.
"For Romantics, the sublime is a meeting of the subjective-internal (emotional) and the objective-external (natural world): we allow our emotions to overwhelm our rationality as we experience the wonder of creation. ... Because the sublime is emotional, it is traditionally considered something one must experience alone."
Hamnett, I thought was very good. Epic? Mighty? Maybe but I'm not sure. Is it deep? Yes, very. So why not? Others may well disagree, but perhaps because it is domestic, not panoramic in scale, and timeless but intimate in focus.