You got me curious so I just looked at all the butterfly fish they have listed and soon it became apparent.
The ones that they say can do fine in a 70 gallon (the smallest recommended) all have a maximum size of ~5" AND/OR they are extremely peaceful and easy to care for.
there are only a few listed that fit the above criteria.
The ones that max out at ~5" and they suggest a 125 or larger is because they are either more aggressive or more difficult to keep.
Every other one gets large and can be more aggressive.
The one you are asking about can get ~6" therefore the larger tank recommendation.
also keep in mind that for tangs and butterflies having a long run to swim is more important than overall capacity.
for example I have an 80 gallon that is 60"x18"x18" which would be better than an 80 gallon that was 48"x20"x20". even though they both have the same approximate capacity.
Length of the tank should always be a consideration when thinking of tangs and butterflies, IMO