With a large number of parts in a file, it is easy to find the proper part size (since there is limited number of permutations of likely part sizes, and part sizes can often be assumed to fall on MiB or MB boundaries).
However, for a given upload, as the number of parts diminishes, a number of different possibilities for part size arise and it becomes difficult to have an algorithm that guesses that number and time-consuming to confirm the calculation.
Knowing the part sizes is useful because the algorithm used for ETag calculation in S3 only generates the same value for two identical objects when the payload matches and the two objects were created using the same part sizes during uploads. Otherwise, it generates different ETag values for two identical objects. (There is no requirement in HTTP itself that two identical objects have the same ETag, but matching ETag values is useful for integrity validation).
Is it possible to learn the part sizes that were used for uploading an existing object in S3?