The best way to think about the mining market is that the number of Bitcoins available to be mined each day is _constant_* no matter what the total hashing power deployed is. As a miner your share of that fixed pool of available coins is determined by your hashing power relative to the overall hashing power of all miners. So if you control 30% of the hashing power you should on average get 30% of the coins mined each day. Your profits are then determined by your costs and the value of a bitcoin when converted to the currency your costs are denominated in.

* The constant amount actually changes periodically when the coinbase reward is adjusted downward but that only happens about once every 4 years and historically exchange rate price appreciation has outstripped the reduced coinbase mining rewards. eventually the coinbase reward will go to zero and the number of coins that go to miners each day will be their relative share of the total fees being paid by users transacting on the network. Miners choose which transactions to include in a mined block so in the future a large miner may have some pricing power over transactions because they could refuse to process any transaction with a fee that falls below some threshold.