In terms of total word count I doubt potter was significantly longer than Animorphs as a series.
If they are calling it novel vs novella (or whatever a paper back that size is called) maybe, but as most the books were series rather than individuals, not a fair comparison.
Potter also was not particularly high vocabulary, a high number of big adjectives sure, but not above standard middle school levels. Animorph's was admittedly a grade or two lower, but not so much to remove from the category.
And finally, they both, and all the other books I saw there fell in the same categories at the stories: young adult. I don't recall HP ever being marketed to an older audience, but once the films started coming it was seen as a "family" thing, and since it took them near ten years to come out, they had the carry over, but still a single adult 35 yro in 2007 with Deathly hallows was seen as reading a youth book. Just being common place doesn't up it's level of difficultly