Thank you all for the information and advice. Exporting a video resulted in a normal output.
Your suggestion of updating the display driver prompted me to look at the properties set for Videopad in Windows.
In properties, compatibility, settings, there is an option "overide high DPI scaling behaviour. Scaling performed by:".
I had the "system (enhanced)" option selected from the dropdown menu when the preview was small (1/4 of normal size). When I switched this option to "Application", the preview displayed the normal larger size.
This seems to have resolved the issue.
Many thanks again.