Does selecting bigger backpacks, mean your maneuverability is affected? I know it makes you a bigger target, but I used to only use small grenade pouch and large ammo for LMG, but as economy changes incoming swapped my troops with large backpacks as part of my rush to burn through bronze to get cheap equipment.
Can any confirm you are slower without perks? I would like facts not speculation please 
nope
Nope
I am pretty sure weight doesnt matter at all in game unless it is in your hand, and even then its a bit inconaistant
negative
Weight does technically matter in how much you can carry, but your soldier is such a beast it doesn’t matter. I went the practice rang and pick up everything (MG42, AT rifle, all the ammo), the game told me I could not pick anything more up because of weight. My guy was carrying like 2000 rounds of ammo.
It actually matters for max sprint distance not movement speed, even if the equipped weapon matters for speed too. It also affects stamina usage on jumps (higher cost for higher weight)
If you use up all your ammo you will be able to run further than with the ammo. If you have used grenades, medkit or mines, you will run further than with them on your back.
On a mortarman it is very noticeable: you can run much further after sending 10 mortar rounds downrange.
When carrying AT mines, their massive weight reduces sprint distance a lot, they are very heavy