Refrigerant piping sizing -
First; there are really two things moving through the two kinds of refrigerant piping: refrigerant and oil.
Reduced refrigerant pressure reduces refrigerant density. We want to avoid this by always having the highest density refrigerant which will do the job.
So we want to avoid pressure drops in piping to the largest extent possible. This would seem to indicate that the largest and least restrictive piping would always be for the best.
But there are two other factors: cost and oil flow.
Of course in liquid lines the oil moves with the refrigerant and so pressure drops and cost are the only factors. But in vapor lines the velocity must also be considered. The velocity must always be sufficient to move the oil along and have it return to the compressor or oil separator.
Luckily all these things have long ago been worked out and graphed and curved for us. A really excellent reference for this is the RSES SAM manual's piping sections.
Are you an RSES member? Or do you know anyone who is? The SAM manuals make for Very interesting and informative reading.
The conventional view serves to protect us from the painful job of thinking.