Vader has always seemed so huge and larger than life in the movies that it doesn't really bother me if the figure is a bit oversized, or dwarfs all my other figures. At least as long as he looks right proportionally (unlike, say, the first 15" tall SS Vader that had ridiculously long legs).
And plus HT has already made Kylo Ren way oversized, so we at least have to make sure Vader is still taller than him.
While that last part is true, I feel like they made Vader's helmet maybe a tad too large and Kylo's a tad too small. The scale on lots of these figures are off one small way or the other. Just need to pose them with a figure that's close enough.
Some people focus so much on height, but adjustments solely there can make other things look off as well. Like Rey, it's not just her height, but her width as well that looks too small, she's a 1/6.25 scale figure (maybe everything outside the noggin).