Something that has aided me tremendously in designing and critiquing objects in the world, whether it be bonsai or the product of any other ("artistic") endeavor, is a quality that the Englishman David Pye calls "diversity." Pye, trained as an architect, eventually found his calling as Professor of Furniture Design at The Royal College of Art in London, where he taught for 26 years, the last 11 of which were in furniture design. He died in 1993 at the age of 84.
In his lifetime he authored 2 books which have taught me more about design integrity and what constitutes "quality" as anything else in my sporadic grapplings with education. The Nature and Aesthetics of Design , and The Nature and Art of Workmanship.
Anyway, back to the quality of diversity. The way Pye uses the term is to explain that "every formal element has a maximum and minimum effective range. It can only be 'read'---perceived for what it is--by an observer stationed within those limits." He goes on to state that, "In nature, and in all good design, the diversity in scale of the formal elements is such that at any range, in any light, some elements are on or very near the threshold of visibility..." He felt that threshold of visibility (and comprehension) was very important aesthetically. Furthering his hypothesis, he states that as the observer draws closer to his objective, larger formal elements "drop out and become ineffective as you approach." Replaced, he says, by increasingly smaller elements to engage the eye. Bonsai are most often, though not always, shown in interior architectural spaces. The spatial constraints placed upon both observer and observed by that space favor elements of a certain size. (Between his minimum and maximum "effective range"). Larger trees are more effectively viewed from farther away. As one draws nearer, the larger elements, in Pye's eye (apologies, too good to pass up. It had to be done) "drop out and become ineffective." Unless there is sufficient diversity in scale of all formal elements, the scale shift necessary to be convincing is not present.
Conversely, shohin and mame invite, no, demand close inspection. The physical limitations of display space vs human stature brings elbows into the mix. Smaller scale formal elements demand increasingly fine detail.
The physical presence of bonsai, these tiny living jewels invoking much larger worlds, are invariably placed in relation to our bodies. Whether we are aware of it or not, our confronting anything is sifted through atavistic circuits from "eat or be eaten" days. Is this thing larger or smaller than me? If it falls on me will it harm me or bounce off harmlessly? Most importantly, how close to it do I have to approach to 'read' it? Can I "see" it from here? I firmly believe we, as humans, place more gravitas on objects based on their size in relation to our bodies than we suspect. That fuzzy area of plausibility in bonsai design around age, scale, and more anthropomorphic qualities such as 'tenacity, nobility, dignity' etc. may be more easily relatable to us if it is coming from something that affects architectural space to the same extent that our bodies do.