Possible optimisation to bitcoin propagation One of the main arguments for not raising the block size in Bitcoin is the fact that larger block size means slower propagation.

The most space consuming section of the Bitcoin block is the data of the transactions. Assuming a large portion of the nodes is full nodes, there is a large redundancy in the block data.

When a miner successfully mines a block, he needs to send it to the other nodes in the network. Many of those nodes already possess most of the transactions data that is included in the block. 

Maybe it is better to send a hash of each transaction instead of the data itself. In case the receiving node doesn't have this data, it can request it from the sending node. 

This might compress the size of a block by 5-10 times (depending on the size of the hash) and allow significantly larger block size.
My concern would be that in the case where the recipient doesn't have all the transactions, the extra request(s) to get them would end up taking more time than just sending them in the first place.  Miners are scattered all around the world but are likely to be able to pay for fast connections, so I would expect connections between them to have high latency and high bandwidth.  In such a case it's better to preemptively send a larger amount of data (fast because of high bandwidth) than to risk a request/reply round trip (slow because of high latency).

Also, miners pretty often include transactions that they originated, or were sent to them privately, or otherwise have not been broadcast on the peer-to-peer network.  Consider mining pool payouts, nonstandard transactions, low-fee transactions which the miner has agreed to "accelerate", etc.  So I think this extra request protocol would be needed more often than not.

This could be a reasonable optimization for full nodes who are not miners, and thus don't need to receive the block urgently.  They might save some bandwidth this way.  But on the other hand, since they don't need to receive the block urgently, we don't care so  much about them, and it's hard to justify a major p2p protocol change just for this.