Usually this happens when you're attempting to export a large amount of data to Excel. The component we're using doesn't let go of memory as it's read in and written out to the file being created. You can work around this limitation by giving more memory to the jvm. Find this line in your sqldeveloper.conf file
And bump the 256M up to like 1024M. Save the file, restart SQL Developer and try the export again.
Use the CSV file format. It doesn't suffer the memory management issue.
We're looking to address the XLS performance problem directly in a future release.