Ok so the reason it was returning wrong values was because I'm just plain stupid.
Basically I do something like:
WalkDir::new(source).into_iter().map(|item| item.metadata().unwrap().len())
Except I was writing this:
WalkDir::new(source).into_iter().map(|item| source.metadata().unwrap().len())
See the little source instead of item before .metadata()? The code compiled correctly but I was getting the size of the source directory, which is by convention a cluster's size 
So problem solved, thank you all for helping and sorry for wasting your time 