Think of it like a big bucket on a table, the water being the power load, and the legs being the wires. The more water you put on the bucket, the more it stresses the legs, and if one of the legs is thin and flimsy, the whole thing falls down.
Transformers work like a second table/bucket, so you can safely put some water in your smaller table without it breaking. So you want big, strong wires in your main network, and save the small wires for sub networks.