It’s essential to have a pack (or an assortment of different sized packs) that allow you to carry the equipment you need, comfortably. Whilst the big-name brands in trail running have fantastic packs for sky high prices, I generally find you don’t need to compromise too much on performance with a much cheaper pack. Just check they don’t rub, can be tightened as much as needed for fit, and are large enough to fit everything in in a way that’s accessible.
Many packs nowadays offer a variety of methods to carry hydration, whether via bladder system (out the way, but can be tricky to replenish, and give you little visibility when you are about to run out), or front carried bottles/soft flasks (can bounce around). In terms of sizing I tend to find between 5ltrs and 12ltrs for any day run is sufficient.
Generally, whilst having a smaller pack may seem a better idea, there is nothing more frustrating than having to totally empty a tightly packed bag to find the thing you wanted at the very bottom, vs having a slightly larger bag where everything is more accessible. Today’s bags have so many tightening features, bungees, clips, mesh pockets etc, that even a nearly empty 12ltr pack shouldn’t be bouncing around much on your back.