kald: here's what they mean: pipe is usually sized based on an economic optimization between pumping energy and pipe capital cost (material, fabrication, installation, supports, insulation etc.). That optimization is sometimes dumbed down to a range of acceptable velocities for design. And that range of velocities or pressure losses per 100 ft of pipe is often followed even in situations where the process requires you to throw away pressure energy by throttling. The opportunity to reduce line size and save money in those cases is often not taken by the designer out of (usually) irrational fears about erosion, or simply because the high velocity in the line doesn't "feel right" to them because it violates a rule of thumb they've been applying for years without completely understanding WHY. Instead of reducing the linesize to the true optimum in these cases, i.e. the size which makes use of as much of the available pressure drop as possible without itself becoming a controlling resistance, they use the same rule of thumb for the sizing of a line up and downstream of a throttling control valve as they would on a long line between a pump discharge and a tank.
A control valve is sized to give a certain pressure drop at a certain flow even when the valve is fully open. That pressure drop is (usually) selected so that even when the valve is fully open, at the maximum design flowrate, it represents a CONTROLLING fraction of the pressure difference between point A and point B in the flow of the fluid- often roughly 1/3 of the total pressure difference. That determines the Cv of the trim required. If frictional loss in the inlet and outlet lines are low because of sub-optimal line sizing as noted above, the control valve ends up a substantially larger fraction of the total pressure difference, which is OK from a control perspective because it means that the control valve itself will not have to compete with other pressure losses to control the flow. Usually, a trim with the required Cv can be fit into a valve body which is a couple NPS sizes smaller than the line itself, therefore it is not uncommon (in fact it is NORMAL) to see a 3" control valve in a 4" line, a 4" control valve in a 6" line etc. Sometimes, the required valve will be several sizes smaller than the line because the pressure drop required for adequate control is quite high, i.e. when a valve is used to throttle a fluid from a high pressure separator to a drain or tank etc.
Control valves can be fitted with a range of trims which depend on the size of the body. The required Cv may not be available in a larger valve body, and even if it were, you wouldn't want to spend money on a larger valve body just to avoid a couple reducers on either side of the valve in the piping.