So, if I'm going to eat more than the serving size, what does it matter, right? Here's why: instead of just ignoring the serving size altogether if you know you're going to eat more, establish a reasonable "portion" that you can work with.
Do I need to eat a 310 calorie snack in between meals? Especially if it's while I'm plopped on the couch watching old Monk episodes? Hell no. Am I doing it anyway? Uh yeah. It's Saturday. It is important to prepare a "portion size" on these "bigger than serving size" occasions so that you are still conscious of how many calories you are consuming.