Each fixture in your bathroom has a drain pipe. These join together into a slightly larger pipe. If you have another bathroom that shares a wall, the larger pipe from each will join together into yet another larger pipe. Same thing for your kitchen, basement, or anywhere you have an appliance with a drain inside your home. Eventual these all join together into another even larger pipe that goes outside and out near the curb to join another even larger pipe, the city main. This main will go down your street until it joins with another streets main, in yet another larger pipe. And so on until it gets to the sewer treatment plant.
So basically, you start with something like a 1 inch pipe and by the time it reaches the sewer plant it may be up to 18 or 24 inches. Maybe even larger in larger cities.