Is it by design that it affects radius and not the actual area?

 

E.g. 19 Int gives a "+54% Area of Effect" but in reality a circular AoE has a 54% larger radius, thus increasing the area itself by 137%.

Yes.

 

If this was not the case INT would be pretty useless for any AoE.

 

The description could be more correct. But then it would also be more complicated and perhaps confusing for people who take no interest in math.