Almost always it will be from experimental results or past experience with similar setups.

Tractor manufacturers have gone to great length to reduce not necessary lubricant flow in their complex transmissions that contain gears that can be shifted, hydrostatic components, final drives, differentials and oil immersed brakes with the goal to reduce churning losses and CO2 emissions. Maybe they have published some material on that. Generally speaking the optimum would be to only supply the lubricant to the areas where needed in just the right amount necessary for lubrication and cooling. That is unattainable with a simple splash system and maybe some baffles to direct the oil flow. A circulation system that directs the right amount of oil to meshing gears and bearings would be best - but is costly and would also quite a bit of development and testing to finetune it. For lubrication itself very little lubricant will be needed, but to get the required amount of cooling a much larger amount of fluid may be needed, also depending on whether a heat exchanger is mounted or when just the surrounding air around the gearbox is relied upon.

It all comes down to manufacturing cost vs complexity - a complex design could do with a lot less oil then the more common design. The component manufacturer most times will opt for cost efficiency, purely for economic reasons.