I just did a test. With Camera set to Parallel Projection as @Anssi suggested and the standard top view selected the DWG and DXF exports are both exactly the same size.
Thank you Anssi it was Parallel Projection.
What a rookie mistake. I think its because I was using a different default Template in 2020 than 2018 and I was used to Parallel Projection being the default setting and so didn’t even check.
Thanks again everyone for your help. Much appreciated