I think it
Er...I don't think this is right. 20% smaller? If so, you'd never get it on your head!
While the ATA may be fractionally smaller than say an TE2, you'll experience far more variation in how the pull was done and even more in how it's trimmed and assembled. I've had several ATA, TE2, MKT, and even put it side by side with an RT. In the end the smallest & largest were TE2's, simply in how they were assembled. I have two ATA's now, one is made pretty small and I can't put it over my glasses. The other was made by a different person, and I can easily get it on/off with glasses.
So, YMMV...