I can reproduce this also on win 7-64 in QCAD 3, agreed the 'padding' as you say is absolutely perfect and this does result in a tightly cropped fit when exporting to a bitmap image, maybe a 'padding' border size option could be added to the export dialog here!
For now a workaround you could use is set out below, it`s not perfect but it may help you out for now
1.Set up new layer 'Marker' with color set to something you can see well e.g. Yellow.
2. Working on this new layer 'Marker' draw small circles where you need the border 'padding' to be.
3. Finally select these added circles and in the property editor change the radii to ' 0 '.
4. Export your file to bitmap and you should get a better result
Hope this comes in useful for now.