I could be mistaken, but I believe that size of her claymore in-game may simply be for gameplay reasons, similar to how Ishtar hovers in her idle pose to give the illusion of appearing taller instead of actually changing her model, especially since this path's story video seems to show Grand Master with the larger claymore just like her portrait and transcendence artwork.
It's mostly relative to other depictions of her Claymore in artwork. I'll just reword it.