c) the fuller the blocks are naturally, the less extra space there is , so cheaper to spam (this is why bigger blocks will help fight spam)
i find this statement contradictory.
cheaper to spam means easier to make more transactions.
Lets say that naturally blocks would be 800kb. If you only have 200kb free per block, and a transaction is say, 200bytes,
thats 1000 transactions. Let's say people don't really want to pay more than $1 (they can use paypal or another coin),
that is going to cost $1000.
But if you have lets say 8mb blocks, and 7mb are free , thats 35,000 transactions. So even if transactions are only 5 cents,
it still costs more ($1750).
Obviously that's just one possible comparison but the more full blocks get the cheaper it gets to spam, that is the principle here.