Retracting, the small annulus end takes the pump flow while the big bore end empties. A 100 mm bore with a 50 mm rod expels about 53 L/min while the pump delivers 40 — so the return line is the one that has to be bigger.
The piston moves at flow divided by the annulus area, and the whole bore area empties at that same speed. The ratio out to in is exactly the ratio of the two areas, so the more slender the rod, the worse it gets. Size the return for the supply flow and the restriction builds back-pressure on the annulus face, which subtracts directly from the force you calculated — the cylinder quietly gets weaker rather than obviously failing.
Only on retract, and only for a single-rod cylinder. Extending, the flows reverse and the return is the smaller one. Regenerative circuits deliberately send the bore-end oil back to the pump side instead, which is the whole trick.
Estimate with the rule, then check it against the calculator that models it properly.
Open Hydraulic Force Calculator →Retracting, the small annulus end takes the pump flow while the big bore end empties. A 100 mm bore with a 50 mm rod expels about 53 L/min while the pump delivers 40 — so the return line is the one that has to be bigger. The piston moves at flow divided by the annulus area, and the whole bore area empties at that same speed.