That's not easy to answer, because of the scale mode. I don't know the exact values, nor the exact algorithm. It would be best to ask Ashley for the exact way of calculation. But I would guess, that "inner scale" always makes sure that the scale chosen results in a size equal to or greater than the new window size.
The starting window size of the project is 640x360.
We know the original aspect ratio from that size, which is (y to x)1.77~ or (x to y)0.5625
The new window size is 1024x720.
Calculating y from 1024 results in (1024 * 0.5625) 576
Obviously 576 is a lot smaller than 720, which would show more of the scene. Inner scale prevents that, so let's try the other way round.
Calculating x from 720 results in (720 * 1.77~) 1280
Since 1280 is greater than 1024, it won't reveal more of the scene, but instead hide a good portion of it. This is what inner scale is for, so the resulting size will be
1280x720, cropped to 1024x720
Now to the scale. We have the original size of 640x360 and the new calculation of 1280x720. So the scene is scaled by the factor 2. (You can calculate that from either width or height, since the aspect ratio doesn't change)
But again, this is to be taken with caution. It's just how I would do it. Ashley may have used another algorithm, since he is a professional programmer, which I am not.