I think you'll find that the further down the tone and volume rabbit hole you go, the more confusing it gets. The most important thing to remember is that an instrument is a system of interacting parts. It's easy to talk about changes in very abstract and general terms, but when talking about a specific change to a specific instrument, you really can't discuss variables in isolation, you need to consider the entire system.
In that abstract sense, generally, the two variables that this is usually distilled down to are mass and stiffness. The goal of a soundboard is to take the energy you put into a string when you pluck it, and use that energy to pump air. Working within the ranges that are typical, a soundboard that is lower mass will (usually) sound better (louder, at least) since it takes less energy to get it pumping. Similarly, a soundboard that is stiffer will (usually) sound better because the stiffness helps keep it pumping rather than spending energy deforming a softer material - think of the difference between a tuning fork made from rubber and one made from aluminum. Stiffness helps with good sustain because the energy is transferred to the air instead of being wasted on deforming the structure. So, builders generally favor choices that help with stiffness and light weight construction. The factor that usually limits these choices is durability - will the instrument stay together and not deform under normal use? And then tone comes into play, which is generally a combination of stiffness, weight, and overall construction (things like soundhole size, body volume, etc) which influence the resonant frequency of the instrument and how it tends to emphasize or de-emphasize certain frequencies. The other typical limiting factor is matching those variables to the instrument size and the level of energy available - a tiny ukulele with a big bridge on it may be SO stiff that the strings have no chance! And ultimately many building decisions impact those two variables in opposite directions. It's easy to make something lighter but most changes that make things lighter also make things less stiff. It's hard to make a single change that improves both, so you're often balancing a change that increases one but decreases the other. Sometimes the ratios involved are more important than the absolute values.
Which gets us back to the first point - the instrument is a system. It's a collection of dozens or hundreds of choices. Each of those choices may not be important on their own, but when you add up a given set of choices, you can start to influence the "whole" in one direction or the other. And, changes in one variable may limit your choices in other variables. You may not notice a difference between two bridges that weigh within a few grams of each other, but if the builder is trying to "stack" the variables one way or the other, the overall result can be significant. A given bridge may be designed to work with a given top thickness, bracing pattern, bridge patch (or absence thereof), and so on. On one instrument, you may be able to change a single factor and get a good result. On another instrument, changing the same factor might not make a difference, or might have a very negative result.
That's where part of the challenge of doing bespoke custom ordered instruments comes into play. As a builder, I bring to the table some knowledge of what I am capable of doing, which is (quite clearly!) only a subset of what is ultimately possible. The gray area of the unknown can be challenging when it comes to a situation where someone is paying me to do something. I have a "standard" design with a bunch of variations that I've experimented with pretty thoroughly and would not hesitate to change if someone wanted them a certain way. But there are other parts of the design that I know should not be changed, because I've tried them and it didn't work so well (or changing them would be an obvious issue so there was no need to try and prove it). And there are still other parts that I have no idea what the result of a change would be. If someone wanted a custom uke with something from that second set (bad change) altered, I would say no - even if it meant losing the sale. If someone wanted something from the third category (total unknown) changed, I would still probably say no, unless there was a very clear level of trust with that specific customer and/or some sort of "out" clause in terms of managing what happens when the instrument turns out undesirable to either of us.
I think that's the long winded way of saying, trust your builder! Ukulele bridges are often so small and so light that they are within spitting distance of failure. And, to make things harder, ukuleles generally have very little string energy available, so it's VERY easy to over-build and ruin things. Going to a smaller or weaker bridge might result in a failure, Gong to a heavier bridge might result in a uke that's quieter or has less pleasing tone. Since we don't really know his design, we can't really give you a straight answer. If he wants to use a specific bridge design, it's probably a good idea to do so. Or, if you really want a certain bridge, the best choice might be to find another builder you can trust who has an overall design that's compatible with your needs.
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