1 MB Block Size as a Anti-DoS Attack Measure
The DoS the block size limitation is trying to prevent is a miner creating a huge block of garbage data that now every single Bitcoin miner must download, validate, and store the entirety of. Imagine if some miner decided to create block after block of 1GB of garbage, and now every single Bitcoin node in the world must store and validate all of this garbage. If it were possible, this would be in the best interests of larger miners to do: smaller miners, who don't have the ability to download and validate these giant blocks of garbage as efficiently, will not be able to keep up with mining.
Also, people do attempt to submit spam to the blockchain. Trying to do so to the point of blocking out legitimate transactions will be quite expensive, considering you would have to pay transaction fees for every &quot;spam&quot; transaction you want to send.