In my experience, between the top strap, side compression straps, and the comPACKtor system, the Pinnacle adapts nicely to different size loads. I've almost always used my Pinnacle packed in burrito mode with a foam pad, and by varying the diameter of the pad and compressing insulation to different degrees, I am able to find a comfortable setup as food volume decreases over a trip, and amount of carried water changes through various stretches.
I think the primary concern with smaller loads in the Pinnacle would not be discomfort, but rather the inconvenience of excess weight and fabric. At this point, the Pinnacle is the only frameless option I own, so I work with what I have. The only time I've felt inconvenienced by its size is on a day hike when I used, at most, a quarter of its volume, probably a lot less. But my gripes would apply to any large pack being used to carry a hydration bladder and a few energy bars!