Yeah, this has to be a mistake. I doubt they would have let slide "fishy fish" or "mommy mom," but somehow "elven elf arc" made it to the final draft.
Unless...maybe the author felt a need to assert that this momma is not a freak. In this special world, "giant" and "elf" don't form an oxymoron. She's within the norm among elves in this thick chick universe.
"She's a very elven elf! You'll see! I'll hold a convention!" As in Charlotte's Web, the farm hand said of Wilbur, "that pig is quite a pig" (paraphrasing). So this author might be going further, defining their world's elves with this giant elf as the exemplar.
This doubling down on "elf" recalls Humpty Dumpty: “When I use a word...it means just what I choose it to mean — neither more nor less." That's an author's prerogative. As a reader, I'm fine with it.
But it still seems a little off. Mommas should come in all shapes and sizes. Some are slender. Is this just a world designed to suit the author's fetish? Or will there be a comically slim mom, maybe an orc, oni, or dwarf, to round out the harem?