and the one out of probably 100 (?) that survives these conditions will more likely to respond better to any changes in his environment.

one way or another the way they were fed, the way there kept when they're tadpoles, babies, and so on as well as genetics will influence the way they will respond to anything in their life, it will affect their adult size, lifespan and other factors, so to get more objective results that part has to be constant and since it is not possible to achieve unless clone them, control groups have to be very big.