I can export some fairly large queries in SQL Assistant JAVA edition, but there appears to be a soft limit somewhere around 20 Meg where it just cuts off and creates an empty file. The results appear to be accurate, the export unwilling.
You've discovered a bug. The problem turns out that the code from Eclipse that does the export tries to build a string in memory that contains the entire result set. It then writes that string to the file. While building the string for a very large result set the application runs out of memory and the export fails. It had created the file before the failure, thus the empty file.
We will report the bug to Eclipse. We'll try to work around the problem in our next version.