Just read the forms. They only ask you to calculate family size as of when you fill in the app. There is no calculation provided for previous years. The number you come up with based on today's family size, is the only one they ask you to use.
So if you think you should do your own calculations in previous years, then you should be finding the reference that they want old family sizes in previous years since it doesn't indicate that anywhere that I can see. And based on your logic why is your family size 4 in previous years, shouldn't it be 2 since you weren't sponsoring your parents in those years?
IRCC wants to see you meet 3 years of consistent income to meet your family size today, that you will be financially responsible for as part of your app. Why would they be satisfied if you only met old family sizes that have nothing to do anymore with your situation today? Logically this does not make sense for requiring the 3 years in the first place.