Thank you for your help.
1 MB was the maximum Bitcoin block size until 2017, not the size of each block. In the early years there were few transactions and most blocks were almost empty.
After the august 2017 segwit soft fork, the way how the transactions size are measured and the maximum space available has changed, now blocks can be bigger than 1 MB. If you are interested, you can find here how the things works now: https://en.bitcoin.it/wiki/Weight_units
This is the historical chart of Bitcoin average block size
currently the average block size is about 1.33 MB
Edit: I have updated my answer to take into account the changes introduced with segwit soft fork.