They are not sending the whole candidate block, only block header is needed to mine nonce, to use timestamp as a nonce, to use block version as a nonce. Later, updating at least the coinbase transaction is needed (or changing the order of transactions, replacing them with those with higher fees, things like that). From nonce you can get 32 bits. From block version you can get many bits, something around 20 I guess. From timestamp you can get around 10 bits. All of these three fields combined will give you around 64 bits that you can change. Blocks are produced every 10 minutes and they have around 80 bits. That means 64 bits is something around bare minimum you can submit on Stratum, values below that will be useless from pool's perspective.