As I can see, you set the scale of the emitter-node to 0.3. This means all its child-nodes (particles) are also being scaled to 0.3. With the targetNode = self the created particles are not children of the SKEmitterNode anymore but of the node that is being represented by "self" (your scene). If your scene has the default scale of 1.0 than the particles will scale will also be 1.0.
Changing the size of the emitter with the particle-editor and not programmatically will remain the size.