The dlist file is a list of file paths, with per-file information, and it grows with the file count not file size.
You would normally see it upload as one of the last uploaded files. It has dlist and date in file name:
2020-10-02 14:21:24 -04 - [Information-Duplicati.Library.Main.BasicResults-BackendEvent]: Backend event: Put - Started: duplicati-20201002T182004Z.dlist.zip.aes (40.26 KB)
2020-10-02 14:21:25 -04 - [Information-Duplicati.Library.Main.BasicResults-BackendEvent]: Backend event: Put - Completed: duplicati-20201002T182004Z.dlist.zip.aes (40.26 KB)
Above was a small dlist. The largest I can recall was 5 GB. Can you please try looking at a retry log?
If you don’t want to set up the log file, you might be able to use About → Show log → Live → Retry, however it will probably be less reliable and harder to handle compared with the log written into a file.
If you decide your file count makes a dlist larger than your NAS can accept, then do smaller backups.
This is all also a guess until some log information comes out showing how far the backup had gotten.
You can also look at the destination folder to see how much got there. Is it plausible for source size?
Actually, this sounded like a problem on an existing backup, so maybe there are some dlists already.
Can you sort destination files by file size? That would show how large a file you’ve managed to write.