The intensity of disk usage in terms of writting
For every filesystem the less data was written to the disk after the file deletion, the more chances to recover these files.
On the NTFS for small files (typically less than 700 bytes) the MFT (Master File Table) record stores file data along with other file attributes.
Such files are called resident.
Even if you overwrite all the disk capacity with non-resident (large) files, typically resident files can still be undeleted.