If you are a block creator, then you have an incentive to create the biggest possible block. Why? Because then, you can pick some simple, deterministic algorithm, and generate terabytes of always valid transactions on-the-fly, then let your miners work on that header, and send mined block to other nodes. Then, you can start producing some next block, on top of what you created, while other block creators will try to validate what you submitted.
You appear to assume that other miners will wait to finish validating your new block before they start mining on top of it. Many (most?) miners will instead just assume that the new block is valid (who is stupid enough to expend so much effort on producing an illegal block?) and immediately start mining on top with an empty block, while validating in parallel. Once validation completes they can then continue mining with a non-empty block on top.