There is a file limit on ftp, we had to enforce this as so many decided to upload large files and use us as a storage service for personal files, often copyrighted files, which is a violation of TOS and in the case of copyrighted files, also US laws.
Try using the upload option inside the cpanel filemanager, that have an higher limit, as the abusers are less likely to go about and use that since it isn't just as easy as using ftp (no drag and drop).