Enigma is right in that if the value of a bitcoin never rises above the cost to mine/bitcoin, there is value constantly being taken out of the system. Is there economies of scale to be had in mining?
Mining is ultimately limited by computer technology. The most efficient miner available for pre-order is 20nm tech and the most efficient miner released is 28nm tech. You're also limited by how much that tech costs. For example Intel is spending billions on their new 14nm foundries, and there are very few foundries that produce 20nm semiconductors, so you have to deal with the fact that even though better tech is more efficient, it has an exponential cost up front.
I don't think Bitcoin will see miners smaller than 20nm for at least a year, maybe two. Honestly none of this even matters, Bitcoin's price isn't driven by mining. Mining is a lagging indicator, not a leading one. More people mine bitcoins because they're worth more, they're not worth more because more people mine.