I see. Yes, the filesize is unusually big, approx 10 times bigger than expected (after quickly creating a new file with 4 Bass Clarinets).
I couldn’t find any solution, but I assume it has to do with your process of deleting the unwanted players from a template. Is there something, a playing technique, etc., which is linked to the trombones? Dorico wouldn’t let you delete it then maybe. But this is just wild guessing. Does the problem appear, if you redo your whole process by again duplicating your template and deleting unnecessary instruments?
Someone more advanced in looking into the .dorico file might be able to help for sure.