lower bound of block size towards increased speed of propagation
There is a lower bound: Blocks must include at least one transaction, the Coinbase. ;)

The advantage of a smaller block has been somewhat overstated in the past, as it usually doesn't matter much, especially since the relay network got improved with Compact Blocks, a compressed block format that significantly reduces the peak bandwidth required to propagate blocks.

Basically, for a smaller block to have a relay advantage over a larger block the two blocks would have to be found within less than 5 seconds of each other. This only occurs once every two to three days and thus is hardly a significant quantity to optimize for in comparison to the chance of even finding a block to double spend with in the first place.