Ok this is a physical one.
Temperature is just the total speed of molecule vibrations in an object.
Vibrations are movement. Movement needs energy. All things thrive to be in an energy neutral state, energy always disperses, disbalances are always balanced out.
This means that the natural state of objects is 0°K, the lowest temperature possible, no movement.
That is why you should fill up your fridge and freezer! The only energy you need is for removing heat that comes into the thing and would in turn transfer to cooler objects and warm them up.
But keeping things cool itself doesnt need any energy 🤯
And if you heat it up then less air comes in, and the incoming air will be cooled down faster (energy balanced out between low density air and high density things). So the overall temperature doesnt spike as much and less needs to be transported off.
If it’s directly touching the walls it will decrease the efficiency of the freezer since initially the rate of resistance to heat transfer could have been represented by L/KA+ 1/hA with L being thickness of the wall of the freezer K being the thermal conductivity of the freezer wall and A being the surface area and h being the convective heat transfer of mostly stagnant air. In that case the effect of conduction through the freezer wall is a worse insulator than the transfer of heat through the air but by touching the item to the freezer wall you are trading that 1/ha for another L/KA with a negligible L so the resistance to heat transfer is also negligible.
That’s a lot of terms but you have experienced this before if you have a pan on the stove and you hold your hand above the pan it will feel significantly less hot then if you place your hand directly on the pan. If you have someone put a fan over the pan so there is a flow of air and then put your hand in the same place it will feel warmer since now you have air movement which increases your h term but it will still be significantly less hot then if you touch the pan.
I still might not be explaining this well but the math for it comes from the resistive model of heat transfer that while it has a lot of reciprocals I still find it to be the easiest to get an intuitive idea of what’s happening
In summary though from everything.
Empty freezer bad because the air inside leaves whenever you open it.
Very full freezer bad because it blocks flow of coolant causing hot spots and cold spots so needs to be kept much colder to make sure the hot spots don’t get too hot which causes more losses
Touching freezer wall bad because accelerates warming
Mostly full freezer good because the items work as a heat sink helping to keep a regulated temperature while allowing proper flow of coolant