The larger line is called the "suction" line. It has this name because the refrigerant in this line is being "sucked" into the condensing unit by a compressor. The refrigerant in the suction line is in a gaseous or vapor form. The refrigerant is then pulled into the compressor in the condensing unit, where is compressed back to liquid form. When the gas is compressed, the heat it picked up from the inside unit becomes apparent. Next, the hot, compressed refrigerant travels through the condensing coil, which disperses the heat by a fan pushing cooler air through the fins of the coil. That's why you feel hot air coming from your outside air conditioning unit when it's running (in most cases).