In the last few days playing with custom export I ended up with a map containing small.at5 medium.at5 large.at5 while I usually get a large.at5 file only.

Why?

That's a little curious but without seeing exactly what you were trying to export I can't really shed any light on it. Typically you only get small and medium at5 files when displaying vector maps at lower levels of zoom, and we always use a maximum value that is within the "large" level.

Exported this way (don't consider the project name in the export dialog, please):