Different type of feats. Even if it was a bus that was thrown to Diana, she would slice it in two just the same. Of course Vision wouldn't have problem replicating the same feat against a car instead of bus either.
I think I have to agree that these are two different types of feats. Yes, the bus is bigger than a sedan, but as several people here have noted, Vision's is a durability feat more than it is a strength feat.