Now, all blocks are sent in uncompressed form. They can be compressed by each node on their own, there are many ways of shrinking that data that could always be made losslessly, for example nonce can always be placed as four bytes in previous block hash instead of zeroes (because there always will be zeroes, 32 zero bits are mandatory). Many things are repeated in the blockchain, so if we create some new nodes, they have to download everything and validate uncompressed data. As validation take some time anyway and sometimes storage or network bandwidth is limited, maybe it is a good idea to send old blocks in compressed form, for example by making such "compressed block dump" every 210,000 blocks. In some edge cases where halving is nearby, we can wait 2 weeks before we compress blocks from last halving, just to make sure we are safe from reorgs just before or just after halving.