Hm, CONFIG_LFS (large file, over 2gb, support) is enabled in our busybox config. If you use a proper file system, likely a busybox bug.
Could you do the following:
- build the latest busybox with LFS and the ftp applet
- check the bug is present there as well
- if it is, please report it to the busybox.net bug tracker
edit: do you mean the ftpget/ftpput of busybox, or the "ftp" client from coreutils? The coreutils client is included in the base.