id say the difference lies in environment, with the player shrunk, everything around you is huge and unfamiliar, and the focus is moreso on the player/POV being tiny.
with an actual giantess, its only her thats enormous, and as such she takes up more of the focus of the scene rather then environment itself.
if that makes sense? not to mention an actual giantess also allows for mass destruction, mass vore and the likes a lot more easily then having to shrink everything down first.
it fills a different side of the same niche pretty much.