The chances of finding a block relate to the proportion of total mining power that you contribute, compared to the network as a whole. For example, suppose you are mining with 1 GH/s and the rest of the network is 99 GH/s. Your chance of finding a block is:
1 / (1 + 99) = 0.01 = 1%
If you double your hashing power, then you have 2 and the rest of the network still has 99.
2 / (2 + 99) = 0.0198 = 1.98%
The reason it's not exactly 2% is the total network is now 101 GH/s, not 100.
If you control 40% of the network hashing power, things get a bit different:
40 / (40 + 60) = 0.40 = 40%
After doubling your hashing power:
80 / (80 + 60) = 0.5714 = 57.14%
As you can see, if you have a significant amount of hashing power then doubling your effort doesn't double your chances of finding a block. However, for small mining efforts the relationship is very close to linear.
This is only an approximation, appropriate for small mining efforts compared to the total network hashrate. – Greg Hewgill – 2014-02-10T19:30:25.480