i think i understand what you are asking: Why is one line larger than the other? The answer is this: The large one is the vapor line, the small one is the liquid line.
During cooling season, liquid is fed indoors through the small line at high density and pressure, and is expanded in the a-coil. The expanded cool vapor is sucked back through the large line to the compressor after absorbing heat. This cool vapor helps keep the compressor from overheating. The large diameter helps keep pressure drop to a minimum.
During heating season, gas is fed indoors through the large line at high pressure and low density. When it reaches the cool a-coil, it shrinks into a liquid as it releases its latent heat. The large diameter allows the compressor to supply lots of superheated vapor to the hungry a-coil. The shrunken, cooled liquid returns to the outdoor unit through the smaller line at high pressure and high density.
No matter which way the refrigerant is flowing, gas is speeding through the large line at low density, and liquid is crawling through the small line at high density. A much larger volume of gas must flow than liquid, around 1000-2000 times more, so the gas line is larger.
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