The guide face in the picture above, please check the position of the pupil.
App try to draw a pixel over there. Mask depicts wider than the frame of the bicycle.
If you want to use a small brush size, pinch to zoom. Then you can use a small brush.

This is not a photo filter. This is a tool for pixel art!
If you want a great result only for you, please think about what you want to create.
Work is not copied. Is created.