I think it tries to pull it first to a temp file. And a path in /var/home corroborates that because 65% of 95 MB = 61.75 MB = free space on the /var mount point.
As a workaround, I'd suggest starting a shell, navigating to /var/tmp/ and then using the ftp client directly to download the file.
Interestingly, a quick search of JunOS documentation shows a multitude of example for uploading files to FTP, but not for downloading ... Go figure.