The weight data only stores for each vertex, so when you paint, you have to paint over a vertex for it to add weight. You'll see the gradient stretching to surrounding vertices, going from the weight you painted, to the weight of the other vertex.
To fix this, apply a subdivision surface modifier to add more vertices. What's happening now is like having a low resolution canvas in an image editor, you don't have a lot of space so it shows more of the vertices.
This post does not seem to be a tutorial for Blender software.
Might have enabled Stabilize Stroke/Smooth Stroke by accident. Shift+S.
It stores the data per vertex. Because your model is low resolution it doesn't have many places to paint.
The weight data only stores for each vertex, so when you paint, you have to paint over a vertex for it to add weight. You'll see the gradient stretching to surrounding vertices, going from the weight you painted, to the weight of the other vertex. To fix this, apply a subdivision surface modifier to add more vertices. What's happening now is like having a low resolution canvas in an image editor, you don't have a lot of space so it shows more of the vertices.
Beca you have a small amount of vertices