I still think itachi could use it on a larger scale, but Sasuke using it on the Hachibi would probably be the argument for this but you have to look at both of them and compare.
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Itachi's fully covered Sasuke's large scale katon(from his side) and Sasuke's covered the Hachibi, yes, but it wasn't fully covered. It was just small flames covering a bunch of different parts of a large object.
So in my opinion, Itachi's use of amaterasu is powerful and Sasuke's is skilled. It's like in boxing(Yes i reference boxing a lot, but I'm a boxer, it just makes a good analogy for me) when one person[Itachi] Can throw very hard punches and another person[Sasuke] has amazing foot work and dip game(Head movement- Dipping dodging etc) and just is an all around amazing person, naturally talented in what he's using.
Hopefully this made sense.. Probably not.