Details in https://collada.org/public_forum/posting.php?mode=reply&f=3&t=2913
The X and Y magnifications are simple scale factors, applied to the X and Y components of the orthographic viewport. As such, if your default orthographic viewport is [[-1,1],[-1,1]] as in OpenGL and DirectX, your COLLADA orthographic viewport becomes [[-xmag,xmag],[-ymag,ymag]]. This gives an orthographic width of xmag/2 and an orthographic height of ymag/2.
I believe the last sentence should say:
This gives an orthographic width of xmag*2 and an orthographic height of ymag*2.