I ride my bicycle with a 30 liter 0.6kg short back backpack. The compression straps make it almost disappear when empty, even on rough terrain. It is not bulky in any way. The hip belt stabilizes it even when running and without bothering to clip the chest strap.
What benefits do backpacks smaller than that (e.g., runner backpacks) provide? For any environment e.g. mountain biking, running, racing both, hiking, climbing, commuting.
I see a few benefits - and I sometimes hike/run/ride with a 40 litre daysack, a camera backpack, or my hydration backpack (about 10 litres, for MTB and running mainly). I also sometimes use my Altura Morph Versa 16 litre pannier in backpack mode and even my 60--75 litre trekking sack.
I see a few benefits:
Bigger backpacks intrude into your field of view when you look over your shoulder (riding, road running, even wildlife hiking). The bikes I ride with my bigger backpacks have mirrors or this would be more of an issue. The camera backpack avoids this by being narrow at the top, but the Altura, when well-stuffed, does get in the way of shoulder checks.
That also makes for more drag on a bike, whether you're trying to go fast, or just bringing the shopping home in a headwind. And tucking helps less if the pack is bigger; even empty they can catch the wind.
Similarly when on narrow trails or paths, a lot of backpacks have pockets sticking out the sides. That's another thing to snag on vegetation. I choose packs meant for climbing, which are narrower than my shoulders.
A smaller backpack removes the temptation to take too much stuff. A lot of them keep the bulk fairly low, lowering the centre of gravity (pear-shaped rather than almost a cuboid, or worse, a cuboid with additional high pockets )
There's some correlation between small packs and hydration bladders, if you like those (I use one on the MTB and running but I'm not all that keen).
Compression straps work best if a lot of the contents is soft. With a few big solid lumps they don't always stop some things bouncing around. My tall 40 litre daysack has 2 compression straps on each side (as does my trekking pack), so there's quite a gap between them. This is noticeable on the MTB and even more so when running or scrambling. Note that empty isn't a challenge for the pack, half full is - for example you've put all your warm layers on and only have tools, a bottle of water, and a camera in there.
Smaller packs tend to have stabilising waist belts (or none at all; I added one to my hydration pack). Hip belts are bulkier and more solid, with bigger buckles, as they're meant to take quite a bit of weight when hiking. Combine that with them sitting at hip level, where cyclists are usually bent over and hikers aren't, and you get a recipe for discomfort. But most packs in the 30ish litre range have waist rather than hip belts. The 40 litre daysack I use for shopping (in addition to 1--2 panniers if on the bike) and riding to hikes has a big hip belt, meaning I can hike with next to no weight on my shoulders. I have to do the hip belt up quite loosely even on my hybrid (fairly upright posture) or leave it undone on my tourer.
Over about 40 litres (in a tall narrow design) some form of frame is very useful in keeping the load steady. But with a formable or adjustable frame, you'd set that up for one posture (e.g. hiking) and it's less comfortable in a different position (on the bike). Both my 40 l daysack and my trekking sack have wire-in-foam back panels that form really nicely to the shape of your back but are rather warm.
Runner "backpack" to me is sort of a bad name. They look like backpacks but the main point is the front access, either front pockets or at least side pockets that are reachable. They're not a smaller backpack, they're an alternative to a waist pack/belt pack, so they have benefits like being able to get your stuff out without stopping and taking the pack off. The "backpack" part is either combining water storage with the front pack part or just bonus space.
...running!
I have a 2L camelback that I've used during marathon training for a decade now. It has enough water capacity that on a long training run I finish drinking right as I finish my run, but not too much that I'm lugging around a bunch of extra unneeded weight. It straps close to my body so there's absolutely no jostling while I'm running, just a little bit of swishing of the water, but that's minimized by its slim but tall design.
It's got two zip pockets and one open pocket. I store all my running gels in one zip pocket and the trash in the open pocket. Then I store my phone, moleskin, bandaids, & keys in the second pocket. All in all, it's about as full as it can be, but it weighs <5lbs fully loaded.
Before I got this backpack I would drive out to various spots along my running route and plant a water bottle + gel. I always had to hope nobody had taken them and I'd be left at mile 16 with no water. It also meant significant forethought, because I had to map out my run and find good spots to put the water. I couldn't improvise day of, or I'd be stuck without my planned water breaks. Lastly, I had to drive out after the run and collect my trash.
Any larger bag would provide nothing of value for my goals and just force me to carry extra weight.
Smaller backpacks carry less stuff and therefore weigh less, allowing better ventillation.
My commute often involves a laptop in a backpack with some miscellaneous items, and I routinely sweat through my clothes and into the pack's material, so the smaller the pack, the less you carry and the better your body can breathe.
Where possible, moving the weight to the bike helps, though this isn't a solution for runners/tramping etc.
The advantage of small backpacks by running is that they are more compact and closer to the body. The most problem with anything you carry on you while running is that it jumps on you, which is annoying, affects negatively your balance and slows you down.
When you bike, you have effectively no vertical movement, when you hike, you have slow vertical movement, when you run, you have a strong vertical component to your movement. If anything in your backpack is loose enough to be able to move vertically, it will jump while you run. Therefore, the best option is to take as small backpack as possible.
Most likely you won't see profits of a very small backpack if you bike or walk.
