Let's reduce Jill's ratio to understand better.
45 chocolate chips for 9 ounce. Let's figure out number of chocolate chips PER OUNCE.
That would be:
45/9 = 5
That's 5 chocolate chips per ounce.
Now,
Costco uses 100 chocolate chips for 18 ounces. Let's figure out the number of chocolate chips PER OUNCE.
100/18 = 5.55
That's 5.55 chocolate chips per ounce, for Costco.
That's more than Jill's recipe. Jill will think Costo uses more chocolate chips per ounce of dough.