You aren't being stupid but it sounds like your letting the product line do the choosing instead of your need. Sit down and actually pack up all the things you need to take and see how much space that takes then pick the bags. Get a couple cloth shopping bags and see how they fill up, that's about 60liters I'm guessing.

I'd rather underpack carrying only essential gear and buying a Hawaiian print shirt if that's what I really want, than carry more items than I absolutely need only to use them once a week.