It depends what you’re planning to use the said bag for. For example, if you’re planning to go on an expedition, then you’re gonna need all the storage space that you can possibly have, while still keeping things light. If you’re looking for bags to carry during an ultra-race, weight is something to consider – you want to go as light as you can whilst carrying what you need. In that case, a smaller sized bag, with practical features that’ll be user-friendly on-the-go is what you’ll need. There are a million ways to strap extra stuff to your bags; so as long as what you 'need' is stored safely in an appropriate sized bag for that, you should be good to go.