Mining BTC is the same like buy lottery ticket.

Let presume 1 TH/s mining machine is a lottery ticket. 100 TH/s  mining machine are 100 lottery tickets.
1 PH/s mining machine seem got 1000 lottery tickets.  Next is how many numbers on a lottery tickets?
risk/reward?  Cost/Profit on a long run? 

1 PH/s mining machine has a chonce to hit BTC during 20 days run. Less than 1 PH/s mining machine
got tiny or no chance to hit BTC during 1 year run as expectation.