the usb output volume depends on the mixer levels, therefore balance in the screen as close to max as possible and for the vast majority of the time this is enough to get near the top of the bar graphs.

Alternatively normalise the file after the recording is saved with a wave editor; for very many years I've recommended the free Audacity program here and on previous forums.

The default output level is a compromise to avoid clipping since a professional player with some of the more dynamic sounds on high sensitivity touch can easily produce very high peaks compared to the average level and these will clip if the output is too high. An amateur player tinkling along to styles does not produce such a dynamic range, and clipping is a very harsh distortion to be avoided at all costs.