When adding new content to the FAST Search backend, you find that the %FASTSEARCH%\data\data_index size has expanded to all available disk space, and your crawled content may not be fully searchable or indexed. FAST Search stores its raw crawling data on the hard drive as it is indexing the content. Then as the processing jobs run, it adds the records to the SQL database and removes the data from the hard drive. So, the reason for so much disk space needed on the FAST server is for temporary use during crawling.
Refer to the below article to solve this issue.
FAST Search index servers out of disk space after or during a crawl