It could be that at smaller sizes that momentarily the position of the airbrush is catching up to the location it's rendering, so it manages to squeeze in another 'dab' of paint before moving on.
If you need a consistent stroke, turn off the 'autoflow' toggle. That will ensure you get a flow consistent with the speed of movement of the tool.
With 'autoflow' turned on, ArtRage swaps between applying paint continuously (when the brush is stationary or moving slowly) to 'filling in' if you do a fast stroke. I'm guessing that at the brush size you've chosen, you're moving at just the right speed, and updates are happening at just the right rate for the one extra dab of airbrush to hit the end of a short burst of filled-in stroke.
If consitency along the stroke is important, switch off 'Autoflow'.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"