It's been my impression that when someone says an ukulele is loud, he or she is using some sort of shorthand for it being able to fully avail itself of the vibration of the strings in the soundmaking process. For example, many inexpensive ukuleles have been described as being over-braced, which prevents the soundboard from vibrating as much from the strings being struck as when there is lighter bracing, and that in turn reduces the volume or fullness of the sound. I think the same would be true with an ukulele where the soundboard was too thick. As Bradford (who, as an ukulele builder, knows way more about this stuff than I do) points out, a player can control volume through the force with which he or she moves the strings, and a quality ukulele will respond, volume-wise, to the touch of the person playing it. If I were to take two (or more) ukuleles and strike the string on each with the same force, I would measure quality by whether the resulting sound is louder and clearer or softer and more muffled. When someone lists loudness as a positive quality of his or her ukulele, what I imagine he or she is saying is that the ukulele was built well enough (among other things, the soundboard is not overbraced and is not too thick) that the sounboard is able to take full advantage of the vibration of the strings and the body is designed well enough (for example, the Kala thinline travel ukuleles have a curved back to compensate for the thinness of the body) to permit the sound to fully reverberate and project. For the same reason, the ukulele's loudness and fullness might be a reflection of the quality of the bridge and the care with which it was attached to the soundboard.