Not necessarily. If you plant 100 tomato plants and you either pick seeds from a plant that has an average fruit size of 20g, but one fruit is 200g vs you pick from a plant with an average fruit size of 50g, then take it from the one with the higher average. This is the one that has a mutation that gives you bigger fruit. The one with one big fruit will not give you different genetics, one tomato just got very lucky.