How does finder figure this directory is 6MB?
Because your directory may contain hidden files. OS X uses hidden files (prefixed with a . dot when using ls -la) to store meta information on the folder (position and sorting of the icons for example). To have a precise breakup of your directory size use

du -h /Users/azz0r/Sites/Personal/php-rest-api


in Terminal. You can limit the depth of displayed files with the -d DEPTH option.

To see the hidden files recursively from a directory use the ls -laR command.

To show hidden file directly in the finder, you could also follow this post Show hidden files Mac OS X 10.7 Lion and 10.8 Mountain Lion.