Since it's magic, the weight is only in reference to Tanya's wielding of it. To her, it feels much lighter. It still hits with a lot of force, though, possibly more because she can swing it harder. It's also not weightless, just lighter to make wielding easier for her as she's kind of tiny and not that strong. I'm also aware that swords are lighter then you'd expect, but I didn't feel like using a full-weight sword of that size for someone like Tanya was entirely appropriate given her size.

Basically, it's magical and I feel like that gives similar benefits to not needing a realistic explanation of it as it does for jedi being psychic. It works by using magic to make wielding easier for the user while still being a nasty thing to get hit by.