blockchain size issue I am running full bitcoin node locally. Currently, the size of the blockchain is around 70 GB.
Blockchain size

Further, I stored all the blocks in a text file in json format. It turned out that the size of this text file is around 10 GB. I am wondering what is the rest of the 60 GB used for?
Your code stores only the results of the getblock call. This returns the block headers and the tx hashes of the block, but NOT the actual tx's themselves. 

Each transaction hash is only 8 bytes, an actual transaction is roughly 500 bytes, so you're missing about 500 bytes per tx in each block. 

There have been about 116 million transactions in the history of bitcoin, so 500 * 116 million ~ 60 billion bytes or about 60 GB that you're missing.