Scaling issue workaround?

[Rik]

I'm not sure what I'm missing here?!

Assuming I've understood the OP's problem.
I think Gary has already answered the question as to what is happening.

The original image is 770x1085px

With the 'Lock aspect ratio' not selected, I entered 1000 into the width. And as expected, only the width is altered. (First pic)
With the 'Lock aspect ratio' selected, I entered 1000 into the width. And as expected, both the width and the height altered. (Second pic)

Select the images and see the measurements.

So, can someone explain to me what the problem is?
I'm using XDP11.