Presumably there were some files on the Destination that were about to be deleted (because not on Source). Otherwise, yes, you'd expect what will fit on one 1TB drive will fit on the other.
The 'available space' warning takes no account of the space that will become available as files are deleted. It's not a road we followed; bear in mind that to achieve total accuracy, we would also need to tally the space currently occupied by files that would be overwritten (given that it's perfectly possible files of the same name may have different sizes).
It's a simplistic test, only displayed during an attended run, in case it's true and you want to investigate / do something about it beforehand. It won't disrupt any unattended backups (e.g Scheduled)
FYG, in both cases, if the profile encountered a file for which there was no room for 'another' copy, it will record that fact in the log, and continue trying, in case any subsequent files are smaller (and will fit in the remaining space). The files are processed as encountered by recursion: there is no sorting of 'largest files first', for example. Nor does it delete orphans first, copy live files afterwards.
The 'available space' warning takes no account of the space that will become available as files are deleted. It's not a road we followed; bear in mind that to achieve total accuracy, we would also need to tally the space currently occupied by files that would be overwritten (given that it's perfectly possible files of the same name may have different sizes).
It's a simplistic test, only displayed during an attended run, in case it's true and you want to investigate / do something about it beforehand. It won't disrupt any unattended backups (e.g Scheduled)
FYG, in both cases, if the profile encountered a file for which there was no room for 'another' copy, it will record that fact in the log, and continue trying, in case any subsequent files are smaller (and will fit in the remaining space). The files are processed as encountered by recursion: there is no sorting of 'largest files first', for example. Nor does it delete orphans first, copy live files afterwards.