Why bigger blocks will bring bigger memory usage 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.