If you look on the transformer in the electric overlay; you see a little battery in the middle (1KJ for the little transformer and 4KJ for the big one).
This means that little tranformers are 1KW consumers. (this battery will fill up when you power up ur circuit).
You use a small wire (1KW max) to supply 2KW consumers. So the wire overload. It s normal. It's consistent.
Transformers are means to be supplied by heavy wire; like on the schema on your screenshot (the big transformers is a 4KW consumers anyway no choice here). Heavywire can be used to link multiple generator.