--usage should be the default

[lambda]

Tools like dutree or du are generally used for finding files to clean up to free up space. The space that will be freed up corresponds to the files block usage on disk, not the file size, which could be smaller due to not using entire blocks, or larger due to a file having holes.

While there may be some circumstances where the file size is useful to know, for instance if transferring to some other system which doesn't support holes in files and does't round to block boundaries, for the most common use cases --usage is the better metric, and it's even implicit in the name, where du stands for "disk usage."