Because a control valve makes its living creating pressure drop and the object is usually to make plenty of it anyway, so why not use a smaller pipe diameter to help it do its job. Besides the fact that a smaller valve is cheaper.
you must get smarter than the software you're using.
If this is for new pipeline design, it should be understood reduced port valves may limit some future "pigging", if desired for various purposes including pipeline condition/damage assessment etc (e.g.
To some extent this practice is a carry over from the past when energy was cheap. As a generalization (dangerous, I know) the higher the pressure drop across the valve the more responsive is its control action. The rule of thumb was to allocate around 30% of the system pressure drop to the control valve, and this was best achieved by making the valve a size or two smaller than the line.
By avoiding rules of thumb and applying more mental energy than pumping energy it is often possible to get away with less than 30% across the valve. And in many applications in the industries where I work the control valves are being eliminated completely and are being replaced by variable speed drives on the motors. In the end everything is driven by economics, and the economics have changed over the last 40 years - and continue to change.
Because we live in a commercial capitalist world in the main. If I can give you a valve which does your duty and is cheaper because it's two pipe sizes smaller then I'll get the order. A lot of time you're trying to minimise pressure drop / velocity in your lines so they are bigger than they might need to be, but a control valve is the opposite. It all depends what your minimum pressure drop is at highest flow - this is what drives the sizing of the body.
Rconner is correct in terms of reduced port ball valves and gate valves / plug valves, but nearly all control valves are not piggable regardless of the size.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
A lot of valves in vapor or gas service are deliberately downsized to avoid an excessive release rate of gas into lower pressure downstream systems in a control valve failure case, which otherwise would have the effect of needlessly driving up relief valve, tank vent and flare system sizes. The answer to most control valve sizing usually is found in the drawings and in the system or connected systems that are often quite a ways away from the immediate vicinity of the valve.
