Small off-topic, I see those boxed backpacks and think of their shoulders. On motorbike the box rests in the back of the seat but here all the weight is in the shoulders.
People who have done some mountain hiking knows that the weight of the backpack should be mostly in the waist, thanks to the waist belt (sorry if I don't use the right term, I've done most my hiking in Spanish ;) ).
I don't know how weight distribution would differ from walking but with some research perhaps they could carry more while staying more comfortable.
The tradeoff is that the higher you carry the bag (i.e. on your shoulders), the more space you have when maneuvering between cars. You can imagine how, if your bag was at the same level as the bike saddle, it would be more likely to get pinched when cycling through traffic. A clever cyclist could even stand upright with the bag on their back to get a few more inches of clearance when need be!
Capacity is still needed, or you will miss out on a lot of deliveries in peak hours, where you load up several orders and then deliver them all in one route and you dont really need to carry whole bag to client/restaurant, just fish up the package.
Also boxy bags allow for bigger packages(pizza, water bottles) and usually help to pack orders in a way that damages them less.
Delivery workers should fuck up their backs because otherwise (1) customers would get their shitty fast food and soggy fries 15s or 20s later, and (2) boxes would eat into the profit of the shareholders.
Reminds me of seeing amazon workers struggle to drag those plastic boxes full of deliveries around the sidewalks of the neighborhood while UPS provides a hand truck. I get it, a hand truck cost more than that 10 cent box they abandon all over town.
In my experience hauling deliveries on the shoulders generally sucks, especially if weight is bigger. hence why most active couriers around my parts prefer to fix some sort of baggage rack to their ebikes.
People who have done some mountain hiking knows that the weight of the backpack should be mostly in the waist, thanks to the waist belt (sorry if I don't use the right term, I've done most my hiking in Spanish ;) ).
I don't know how weight distribution would differ from walking but with some research perhaps they could carry more while staying more comfortable.