A baby only counts in the year the baby was born and the year following the birth. It does not count to the previous years. For example, if a baby was born in 2016 to a new couple followed by a second baby in 2018, their family size would be 2 for 2015, 3 for 2016 & 2017 and 4 for 2018. I have seen a letter from CPC-M where they had created their own table specific to the sponsor.
Ok thank you