It’s not really not the size, most backpacks really are at least 20 liters. It’s the shape. But mine would take 6 1.25 litre bottles, easy. It’s a large quality rucksack. I’ve had it for literally more than a quarter of a century. Something like 26-27 years prolly.

But I agree, 6x 1.25 l bottles in a very basic backpack might present a challenge, because it’s usually not wide enough for one to go sideways on the bottom (mine is tho, for 1.25l, which is uncommon size here tho). Or just put three at the bottom and then three upside down on top of them. Even a basic backpack could do that I’m sure.