They go up against the full weight of the network (the amount of coins held in wallet and are in stake mode), and a portion of them are used to solve blocks and then the cycle of staking your coins begins. So those 50,000 coins aren't a lot compared to the current network weight, so your probability of solving a blocks goes down, and respectively goes up as your balance increase. It essentially the same thing as PoW except without the backend calculations required by hashing power. That isn't to say you wont find blocks, because you will, as the "luck" of finding a block goes up base on the respective age of the coin.