Can you help me figure out why my VM is growing?

I have an M1 MacBook Air. I use UTM to run a Debian 11 virtual machine.

After the initial setup, updating and upgrading, installing Firefox and a few small programs, (I almost called them apps, d'oh), the Debian.utm file size was about 6GB. Now, less than a month later, it's almost 9GB.

The VM size increases after each use. I haven't downloaded or installed anything close to 3GB worth. I have downloaded files, then put them into the trash and emptied the trash, could the trash not be emptying properly?

I have run ncdu, but nothing stands out. When I drill down into /usr and into each large folder, no sub folder is larger than a few 100MB. I can't see any big files lurking.

I've googled using many search terms, but I can't get past results that are about how to increase the size of a VM that is running out of room.

Thanks very much for reading this, any ideas?