To echo Uncut -
The QSA was the QA prototype. It had the trigger of a QA, but the large decocker like an AS. As the decocker doesn't serve the same everyday purpose in the QA as in the AS, the button didn't need to be as large.
IE, in the AS it changes modes. In the QA, it deactivates the striker completely... so, dont want a really bit button for that.
thorn