Today's Bitcoin trivia nugget starts with a provocative statement, followed by a longer explanation.
There are no blocks in Bitcoin.
What is commonly referred to as a block, is a data transport structure. It is used as a simple mechanism for Bitcoin nodes to transfer data between each other. It is a short-cut, a simple efficiency gain: rather than transfer the block header and then transactions individually, it combines the Block Header with every transaction that would be required to fully validate the header, and transmits them in one structure.
The Block Header is 80 bytes and contains the Proof of Work and the Merkle Tree Root. It does not contain transactions, or transaction identifiers, or the "size of the block". The Block Header is tiny. It was designed for scalability.
The communication protocol between Bitcoin nodes is just a communication protocol, nothing more. It is not Bitcoin. Bitcoin nodes can communicate however they like and still follow the rules of Bitcoin and still be Bitcoin nodes.
A "Block" is not a part of the core rules of Bitcoin. There are no blocks in Bitcoin.