Question

Why connecting rod is connected to one smaller end and other to bigger end?

Answer 1

Every component design in an engine has to meet 2 main design objectives. Strength and lightness. For a given material, strength increases with thickness where lightness decreases with increase in thickness. So crank shaft pin must be sufficiently thick to handle the stress and strain of converting linear oscillation to angular motion. To cover thicker crank pin, connecting rod must have bigger end there. At the piston end, the movement is linear, but oscillatory. With sudden change in velocity as piston moves back and forth the entire piston assembly must be as light as possible. This limits the thickness of piston pin and connecting rod at that end.