After installing I didn’t restart. I used the KDE Partition Tool to check the HDD partitions. Then i notice that 5.77 GiB of my 300 GiB HOME directory is already used. Why? I just installed it. What’s the 5.77 GiB being used for?
The /home partition is not only the /home directory, but also the partition table at the beginning of the partition, which will list all the files the partition will hold. That table reserves space from the creation of the partition.
A disk usage tool will show you how much space is used by files.