Imagine you can build your soldiers in a lab somewhere. You can control everything, and you've found a the ideal humanoid form (ignoring potentially better forms here). You could build them in all sizes big and small, but this would also mean that production of weapons and equipment would need to account for various sizes and be more intensive, so using a single size for your super-soldiers would be more ideal.
You could have a few set sizes. Pilots of vehicles and aircraft would likely benefit from being small in size as it would mean they need less space inside the vehicle, but then you would simply have two set standard sizes: "normal" soldiers and "pilot" soldiers that are smaller but all still the same size within their category. For clarity let's say all normal soldiers are 1.8m in size and all pilots 1m in size without any further variation. You can possibly imagine a few more size variants like a heavy duty variant that is larger but that too would be set at a specific height without further variation.
What I want is to have these soldiers greatly differ in size, fir example from 1.5 to 2.5m, but I can't find a good reason. Especially for the kind of large-scale warfare I'm imagining the industrial advantage of building them all one size would be so immense it can hardly be ignored. It would likely need to be in part an evolutionary reason that allows a variable size army to succeed more than one with set sizes.
These soldiers rely for the most part on equipment, so building it into their bodies matching their size requirements is not going to be an option.