When backing up to a remote server, the successful completion of the process relies on many different factors including the type of the remote server (FTP, Amazon, DropBox, etc.), the load on the two servers, the network connectivity between the two, the size of the backup, server limits, and more. The way BackupBuddy works is to create one process to initiate the transfer, which for large websites becomes too long and heavy. The larger the website, the more likely it is to hit various server limits like Apache timeout, PHP max execution time, max CPU seconds, and more. 
On SiteGround servers, we do not recommend that you try to send backups over 150MB to a remote FTP server because that transfer may take too much time and the process will reach the timeouts set to protect the stability of the server or will be killed by our server monitoring system if deemed to be creating load.
In case your backups fail to be saved on the remote location, we recommend that you download them locally and then upload them to the new destination.