Why bigger blocks will bring bigger memory usage for miners? I see the different proposals for the block size and I keep wondering why bigger blocks means a bigger chain ? If the block limit size will rise to twenty-megabyte, it is just a limit, to fill it it requires twenty-megabyte of transactions. so if there are twenty-megabyte of free transactions a block will be twenty-megabyte size but it all still depends on the total size of valid free transactions and now it doesn't even get to 600k in average. yes in time the block will be filled I guess it will take years but it is the natural growth of the system. so my question is why bigger block size means immediately bigger memory for miners ?
Mike Hearn pointed out earlier on the mailing list, Satoshi added a block limit to avoid people mining "troll blocks". These are blocks where a miner includes a Gb worth of transaction output/scripts in the coinbase transaction, or thousands of small transactions. A hard block limit puts a cap on our exposure to this. 

There is no minimum block size, since you can still have empty blocks, so increasing the block limit will have no effect on miners.