Because most the stuff that you put down your toilet is a semi-solid or break down very easily under flowing water. that is why you’re not supposed to put sani wipes or feminine products down the loo because they do not actually break down like paper or poo.
in the same regard, you should not pour grease down your drains either, cuz it solidifies in the cool pipe & begins to clog and build up on the walls
Toilet pipes use 3-4″ diameter pipes. So they’re large enough for solid waste to easily flow through. Most sinks and urinals will use 1-2″ pipe, which is too narrow for solid waste. An experienced plumber will install the piping in such a way that the pipes provide adequate flow all the way to the main line.
The pipes in your house or toilet rarely see something that cannot be dissolved with high velocity flowing water. Most of what might get stuck can usually be broken down with chemical treatments like DrainO or a plumber’s snake shoved down the pipe.
Really get something stuck and household pipes can be easily replaced by a plumber.
Once it leaves the house and hits the city lines, the design is very different. Those are very tough to replace and maintain, so they’re designed to last for 50-100 years.
When the city is installing the pipe, they make sure the soil around it is of good condition and pack the area with gravel so it’s even harder for changes in weather or stuff on the surface to break or bend the pipe. They’re also usually made of concrete or steel, unlike household PVC Plastic and are sized to be 4x larger than they need to be at the time of installation.
If your brother’s massive shit made it through your household 2″ line, then it will sail through the 8″ line to the city sewer. Just to make sure, the city will occasionally go through and “slick line” the pipe with a special low-friction coating, so nothing stops for long.