You say you have "pressure for size" disabled, but your brush size is clearly changing with the length of the stroke. Try selecting a different brush, like "hard round" (if you're using Photoshop.)
It looks like you may have some sort of max/min-pressure curve or something like that, since it stops before being pure black and stops before being a very small diameter. Snakker's suggestion of checking your app settings is a good one
