The pixie never made contact with the greatsword.I think a good example of the problem I have with Graze is the Pixie. Dragons it kinda make sense. They are huge and, probably, hard to miss, but they have tough scales. So a powerful hit from a large greatsword you can kinda see doing some damage even if it doesn't "hit". Right? A mighty blow that is deflected by the scales, but still does some bruising or something. Not significant to a might beast like a Dragon, but something.
The Pixie though is a completely different situation. If you have a Pixie flying around the party taunting them, turning invisible and dodging while pulling their hair. If the dexterous rogue attacks, he can miss. The swashbuckler with the flashing rapier, can miss. The barbarian with the mighty greataxe, miss. But the fighter with the greatsword? CAN NOT MISS. Two swings and the pixie is dead (assuming 20 Str). Doesn't matter if the pixie is dodging, Disadvantage makes no difference. If the pixie is invisible, as long as the fighter picks the right square, it doesn't matter, he can't miss. Two swings and Splat! Dead.
Sure, the wizard can throw his once-a-day Fireball, but that is in the realm of "Nuke it from orbit, it's the only way to be sure." It's massive overkill. But that's what wizards (and sorcerers, etc.) are for.
The greatsword is so big that the pixie had to use a lot of energy to dodge it when the fighter swung at it.