That's 45 percent more than the company's MySE 11.0-203, from just a 19 percent increase in diameter.
So I think that their point is that the energy output goes up more (45%) than the diameter (19%), i.e. it's not linear, which is what "proportionality" [1] seems to mean.
[1] https://en.wikipedia.org/wiki/Proportionality_(mathematics)
1.19 * 1.19 = 1.4161
That's still 4% unaccounted for, so there are some efficiency gains there.
The question then becomes how building materials and energy required to build one of these scales with size. If that is linearly with blade diameter/turbine height, then yes, the efficiency increases quickly the larger you build them.
And the cost savings are likely less about building materials which likely scale at more than quadratic pace with height, but about labour and maintenance. Each trip out to an offshore base is costly, fewer trips made both for building and for maintenance saves a lot of money.
The other main reason to get bigger that‘s not even discussed is increased capacity factor. That doesn‘t have to do with efficiency, but with effectiveness, as the turbine will simply run more days of the year.