It happens sometimes. I had written a multithreaded parser in C++ to parse around 800MB per day so another team could build up the rest of the project based on the data. Someone had thought it'd be a better idea to store all fetched data in XML.

XML is often used when migrating datasets, large and small. Interchange between disparate systems is the very thing its good for.

OpenStreetMap is another example that uses huge XML files. I'm not sure I really like the idea, but it does happen, and if you need the data then you have to be able to deal with it somehow even if you don't like the format.

I had to deal with relatively large XML dumps containing dictionary data.