Well it’s sorted. I generally take photos using two different settings, ‘Automatic’ and ‘Intelligent Automatic’. It seems that, for reasons I haven’t yet fathomed the resulting JPG files are of different resolution. They’re either 4608×2592 or 3264×1840. I suppose I need to work out how to better control this, as both resolutions are probably over-kill. Anyhow, all the photos that displayed small were 4608×2592, changing them in Picture Manager fixes the issue. It did seem to be the Humax that was causing the problem, but happy with the workround.