hey triplets, I just tried importing rather than dragging the file and it works!
Dragging the file in it seemed to create the file OK but then a message about analysing it came up and that's when it reduced the size. When I imported the file it analysed the file before writing it.
Weird.
Anyway, thanks for your help. I'm not sure what was wrong with the file but now it appears to be working so all's good.