A reducer is not a water reducer
Look the word up on its own and you land on two unrelated things. One is hardware: the tapered fitting above, a piece of the run. The other is a water reducer, an admixture batched into the concrete at the plant so a load handles more easily without anybody putting a hose to it.
They have nothing to do with each other. One is steel in the line. The other is chemistry in the load, decided before the truck leaves the plant, and it is one of the corrections a supplier keeps for a stiff load that does not come out of the concrete's strength. If a reducer comes up on the phone, it is worth one second to settle which of the two is being talked about.
What the taper does to the load going through it
Inside the fitting the bore steps down, and the concrete has to squeeze into a smaller opening than it arrived in. It moves faster through the small side and the machine works harder behind it, because every foot of line, every bend and every change of size adds resistance the piston has to overcome.
That makes a step-down one of the places a marginal load shows itself first. Stone that runs large has less room around it there than anywhere upstream, and a mix already coming apart has one more reason to gather and stop. It is a good argument for choosing the aggregate against the narrowest part of the run instead of the widest.
None of which makes a reducer a weak point in itself. It is ordinary hardware in an ordinary run. It is simply where the smallest opening in the line lives, and the smallest opening is the one worth designing the load around.
Why a run would step down at all
The reason is handling. The larger line moves more volume against less friction, which is what you want across the open part of a route. The smaller line is lighter and steers by hand, which is what you want where somebody has to walk the discharge end down a footing trench, through a gate or in a door opening.
So the two sizes are not a ranking. They are two answers to two halves of a route, and plenty of routes really do run open most of their length and then pinch near the end. Which line goes where on your site is a layout question, settled by walking from the spot where the trailer can sit to where the finishers want the first yard to land.