Volume is related to the height of the saddle above the soundboard (up to a point - excessive height can over drive the soundboard and lead to structural failure!). The optimum seems to be around 1/2 inch/12mm.
If you lower the action, you lower the height above the soundboard - thus potentially lower volume. As Timbuck says, you can restore the height by changing the neck angle or fitting a tapered/thicker fretboard (expensive!), or if you're building it, design for the desired low action.
However, there is another factor, playing style. Low action means you can't hit the strings as hard without buzzing. So heavy strummers may have to play more lightly and thus get less volume, even if height above soundboard is fine. Delicate players might not notice any change in volume unless the saddle needs to come down a long way.
But if the height above the soundboard is, say, only 1/4 inch/6mm then that uke will be quieter than it could be.