When you increase the cylinder's bore, you increase the surface area on top of the piston. This spreads the fuel/air mixture's workload and produces more power without taxing the force of combustion. Even with standard cranking pressure, you gain better performance because of the extra size of the combusted explosion.
Conversely, to make more power with a standard-size bore requires that you compress the constrained explosion with more pressure. More compression forces the engine to work harder, resulting in more stress and an engine that wears out quicker.
Making horsepower with a bigger bore has no ill effect on reliability. Running the standard compression means that you can use 91-octane pump petrol. A bore increase equal in thickness to a ten Baht coin turns a regular 250cc into a 262cc.
The power increase from those 12cc costs less and is easier to come by than adding valves, porting, pipe and carburetor modification all together. Big-bores are all about more power for less expense.
Now I can imagine some of you asking "how big can you bore an engine?" Bigger isn't always better. If 262cc is good, it doesn't mean that 292cc is going to be proportionally better. Why not? All engine configurations have a "tipping point." It's the point at which the bore gets too big for the length of the stroke. If you go too big on the bore, you will lose revs and change the power characteristics, and not always for the better.
And there is no magic formula to find the exact moment of tipping. Each engine has its own set of design nuances, and each one responds differently to an increase in bore. The only way to find out is through trail and error. And if you bore the cylinder too big, the wall thickness of the cylinder loses stability. As the piston bangs its way up and down, it pounds the cylinder bore out of round. Round pistons and oval bores aren't a good power match. Even worse, the cylinder will warp and will leak air and oil at a head or base gasket.
If you think how big can I go safely, basically the rule is two millimeters. It is optimum to bore a four-stroke engine two millimeters. Oh, don't get us wrong. You can go bigger. It's done all the time. But it's not always a good idea. Exceeding the two millimeter rule of thumb can be the death knell on some engines. There is a way to go bigger. If you increase the bore more than two millimeters you maybe also have to increase the stroke, but the cost escalates.