Yes it’s true, having boxes of similar size helps us, the mover, reduce the likelihood of damage during your move. Especially long distance moves.
As a mover when I am loading a truck either for a long distance move or a local move the goal for me is to stack your goods into the truck in such a way that it looks even, no spaces, I am trying to avoid having any “air holes”.
In the trucking biz it’s called load shift. It can be as simple as a box sliding across impacting another box or piece of furniture or it can be the entire wall of goods shifting which leads to crushing of goods. Having too many “air holes” in the load can lead to load shift. Too much space between things and when the truck takes a turn or comes around a curve the load will shift. In some cases, not so much in moving industry, but with freight haulers that are shipping very heavy loads, we have seen the entire trailer flip from the weight of the goods shifting from one side to another.
Obviously the first step to avoid load shift is with the moving company by having experienced loaders working on their crews. But just as important is for you to use boxes of similar size. In the moving industry we typically use 8 different types/sizes of boxes. That said however they are all very similar in their dimensions. For instance a medium box measures roughly 18” x 18” x 18” and a dish pack box measures roughly 18” x 18” x 24”. This closeness in measurements allows the loader of the truck to stack his boxes in a very tight order allowing for a minimum of air space.
A good example of boxes of different uniformity would be boxes from the liquor store combined with boxes from the supermarket. Both these retailers utilize different methods for delivery of their products so you will find that especially with supermarkets they have a huge variety of box sizes and types. Not that it’s horrible to use these boxes but it just helps make the job of moving you that much easier. In the end your mover will work with whatever you have and will do his best to avoid any damage.