In unix you mean directory, not folder.
Directories are files, and use space on disk. As the number of file entries in a directory grows, the directory file grows, too. If the directory is actually empty with no subdirectories you can resize it like this - if you own it or are root.
Code:
ls -l -d ./directoryname
rmdir ./directoryname
mkdir ./directoryname
# use chmod to reset permissions as they were before (use the ls -l -d result above)
