It’s possible that there are some leftovers (for some reason), but if you’ve done factory reset then they should be gone. Anyway, I’d advise to try out a tool like WinSCP or MobaXterm and login to the router using them. It’ll allow you to view directories in a nice, GUI-based manner. You’ll probably have to look around in /etc/ and /root/ directories to try and find larger files.
If you’re familiar with CLI then you can search through some particular directories and find largest files in them, but I don’t really have device with similar OS near me so I can’t test the command, but you could run a command like this:
du -a / 2>/dev/null | sort -n -r | head -n 40
and see if there’s some unusually large files. Do note that directories will show up at the top, but individual files will also show up (adjust head flag value if needed), but please share the output here as I’m quite interested as to why this issue is happening at all.
Also, which firmware version are you running? That might be relevant too.