Let's assume that the first page is when they're both are 10 years old.
And let's assume that each year, the presents are decreasing one by one. At first, it starts with 5 gift, subtract 1 every year.
So, the time skip goes from 10 years old, 13 years old, and 16 years old (because the last gift was given when they're both 15).
Now, I don't know much about the genetics of japanese people, and it could just be me malnutritioned from a chicken nuggets diet so I can't compare to her, but there's no way this girl is stacked like that at that age.
Make use of that theory however you want.
Why do I even write this...