Why google play services restore its used space after I clear all data? Before I clear all data, google play services used space is 126 MB.

After I clear all data, google play services used space is 50~ MB.

Before I know it (less than one minute), it restore back to 126 MB.

How is that? my WIFI and SIM data connection is not even on.
Play Services builds its working cache. So, it doesn't essentially need to download anything to become bigger. It starts up, builds its temporary and cached files and you arrive at that size. That said, connecting to the internet can make it more bigger as some content will be downloaded too.