This may be going into the realm of micro optimization that most gardeners could likely ignore with negligible effects to plant health. Yet all of the passionate gardeners that I know are avid experimenters and strive for the closest attainable thing to perfection.
In the 1980’s, Dr. Elaine Inham at Oregon State University (and others) published their research demonstrating the ratio of bacteria to fungi in various soils. Undisturbed soils showed a fungal dominance while disturbed soils contained bacterial dominance. It was found that the woody plants thrived in soils with fungal dominance with the ideal figure around 10 times the amount of fungi to bacteria. (Page 5 of the linked book)
Most important of the fungus is mycorrhizal which form a symbiotic relationship with plant roots that extend root nutrient absorbing capability by a hundredfold. Plants provide carbon-sugars from photosynthesis in return for water and nutrients. Additionally, this category of fungi excretes substances that increase growth of roots below ground accompanied by growth of branches above ground. Also excreted are pathogen-protecting antibiotics.
Glomalin is a substance that binds soils together. Staggeringly, it increases the binding capacity of sandy soils while breaking up heavy clay soils resulting in better soil structure in both cases. This substance is made uniquely by mycorrizal fungi in order to provide firmness to the fungal filaments to reach through soils and deliver nutrients to the plant roots. When this purpose is complete, glomalin attaches to organic matter and minerals in the soils providing the ideal soil structure.
Mulching not only creates the ideal habitat for micorrizal fungi, but the decomposer fungi turn dead biomass into organic acid chains to fuel new plant growth, better known as humus. Fungi that create this humus use lignin as fuel. The best source of lignin is small (7cm/2.5 inches or smaller) branches where the substance has not yet carbonized into wood. This is the exact reason that stems should be used for heating fuel while tree tops are ground coarsely into mulch. The other reason is carbon to nitrogen ratio.
Older wood has a high C:N ratio from 400-750:1 which is much too high for soil microbes. The same holds true for sawdust. Both will still compost, but nitrogen will be tied up within the soil to do so keeping it from nourishing plants. The soluble lignin containing twigs and branches have a C:N ratio of 30:1 which is almost perfect for soil microbes. Nutrients like NPK, magnesium, calcium, etc are found where active budding takes place in green cambium. In other words, anywhere leaves and fruit grow. Thus the smaller twigs and branches are more nutrient rich than stem wood.
Start of my own thoughts:
Here is a side note regarding waste and nutrient management on the farm. The same high carbon content of sawdust and mulch made from stem wood that limits their use in direct soil application is what makes them such great bedding materials for capturing the nitrogen and nutrients in animal wastes. Once composted, this waste/bedding mixture is the backbone of building soil quantity and quality in pastures, orchards and gardens everywhere on the farm.
The next post will be more plant specific. On a closing note, deciduous wood mulches undergo enzymatic decay while softwood mulches follow an entirely different aspect of chemistry. That is why I never recommend mulching fruit trees with bagged and/or purchased mulches which are usually cedar or hemlock in origin.