Finally I was able to compress it! In the end it was a matter of size, as Abs62 suggested.
Here is how I did: (thanks to Tvangeste and wargus for the inputs from this post!
viewtopic.php?f=4&t=1301 )
1. To make it smaller, I tried to convert my file from DOS to Unix EOL both with dos2unix and tofrodos, but I discovered that my file was already in Unix format.
2. Then, again to make it smaller, I tried converting it from UTF-16 to UTF-8, but I had some problems because the file was so big that gedit and other text editors would crash while opening it or while trying to save it. Fortunately, i found a really light and fast text editor named AkelPad (it's for Windows but works in Linux with Wine), which was able to open it without crashing and save it as new file in UTF-8 format. From 2,4 GB it shrank to 1,2 gb (50%!).
3. Then I was able to compress it with dictzip to a small 338 MB .dz file!
I hope this could help other people with a similar problem, so I'll mark it as solved and change the topic to "How to compress large DSL files".