To compensate high suction volume during startup of he pump. the reducer area below the pipeline at the suction stores excess volume of fluid.
big line
your starving the evaporator
The diameter of the discharge line leaving the ac compressor would be smaller than the suction line, because the compressed gas in the discharge line is more dense.
A single suction pump having its suction nozzle on the opposite side of the casing from the stuffing box and having the face of the suction nozzle perpendicular to the longitudinal axis of the shaft
The suction line will be the larger line going into the compressor.
The larger line is suction. The larger line is the suction side which should be on the right side of the compressor......
Discharge side........
The low side is always the big line on the a/c system. The little line is the high side.
There are two coolant lines that come from the compressor. One line goes to the condenser (radiator looking thing in front of the engine radiator). That line is the High side. From the radiator, that line goes through the firewall. The OTHER line that goes to the comressor is the LOW side, or some call it the suction side. Somewhere along that line you should be able to find a refrigerant test port. The low side will only be low when the compressor is running.
When you say too high, just how high is it and what is the high side pressure. It is not uncommon for the suction pressure(lowside) to get up to 80psi on a very hot day. You cannot determine what is wrong unless more information is provided. It could be that your system is overcharged, but without the high side pressure and the temp of the suction line it is hard to give the right answer.
Low side.
You always use the largest a/c line to re-charge the system..It is the "suction side" The small line is the high pressure line.
Low side is the suction side and will have the larger hose attached to it and the high side is the discharge side and will have the smaller hose........
It is common practice to specify the pump suction line one size larger than the pump discharge line in order to increase the Net Positive Suction Head available (NPSHA) to the pump. A smaller suction line the same pipe size as the discharge line would result in more pressure drop in the suction line and reduce the amount of head available to the suction side of the pump. This in turn would result in an operating point closer to cavitation of the pump.
They may be marked "S" and "D" for suction (low side) and discharge (high side). A simpler way would be to look at the lines going into and out of the AC.... the smaller line will be the high pressure line, while the larger line will be the low pressure line. The low pressure line will connect at the compressor inlet, and the high pressure line will connect at the compressor outlet.
If you are asking why the low side is high and the high side is low that would indicate that you have a restriction in the a/c system...........