When it comes to ukuleles and guitars, that's most likely true. The smaller the instrument, the smaller is the size of the part that makes the sound and thus smaller inaccuracies will affect the tone. I'm not sure how big issue that is in reality, though - certainly not big enough to make ukuleles more expensive than guitars.
As for violins, I can speculate based on you've said. It is possible that a violin is acoustically much simpler animal. With strings anchored at the tailpiece and going over the bridge, the whole top works in the same way as a speaker cone. It works the same way for archtop guitars, banjos, dobros etc., by the way. This vibration type is relatively simple to understand and influence with shaping the wood appropriately.
On the typical guitar, however, the strings are tied to the bridge that's somewhere in the middle of the soundboard. They no longer move the top up and down, instead, they cause the bridge to rock, which in turn sends a wave deforming the top. This wave is more complicated, and with all the possible variations in the bracing it's difficult to predict what will happen when you change the shape. In this way, a guitar seems more difficult to design than a violin.
There's also the aspect of amount of energy both instrument receive to produce the sound. Violin has the comfort of the bow being constantly pulled across the string, which produces a constant influx of energy. As long as you don't fail in directing that energy to the top, you'll get an audible instrument and can only worry about the tone. The guitar, however, has to manage with just the initial pluck to feed the sound and sustain. That's a tiny force when compared to the bow of violin. That's why you strive to convert as much of it as you can, and why you need thin tops with complicated bracing instead of a sturdy slab of wood.
I think the main reason why violins seem more complicated (and can be so expensive) is in the players, not the instruments. Tone and intonation standards are completely different in an orchestra hall and rock concert, both for players and audience (not to mention, for a guitar you think about the intonation only during the setup or when you hear it's wrong, but on the violin you think about the intonation each time you put a finger down). That's what is driving the prices, and that's what's making the violin luther's work much more unforgivable than it's in the guitar world, even though the basic instrument design of a violin might be simpler.
That's it for a speculation. I'm not a luthier yet