The reason is because when you send them with the 'scale' option isn't really a true scale.
It just stretches it out in the email so that the size of the image can stay low and the pixels can stay true to size.
To make it stay crisp you must send it at scale 1x
however if you want it to appear LARGE and crisp that means you must then make a LARGE image in pixation (set the numbers high).
This is a decision i struggled with when i decided how to make pixation work.
I decided on 'faking' the size change for a couple of reasons:
1) editing the scaled image after a true scale change would be a real PIA
2) the new image would also be exponentially larger file size
hope this helps, let me know if you have more questions