Possibly.
Do you mean flow rate? Velocity is related to the diameter of the pipe so 20 litres/minute will have a higher velocity through a 15mm pipe than through a 22mm pipe. It's velocity which determines how many kilowatts a pipe can take as the higher the kW the more litres/min. Trying to send too many kW down a pipe which is too small will require a higher velocity, which means higher friction in the pipe, which means more pressure loss round the circuit, which means a more powerful pump. A high velocity will also produce an unwelcome noise.
Pipes should be sized to give a maximum velocity of 1.5m/sec. See, for example,
WB Pipework Sizing Guide, which is based on 1m/sec and
Small Bore Heating Systems.
However, I think you are basically correct. If you set the pump so you achieve the correct ΔT at the boiler, the rad at the end of the line does not get hot because the pressure drop in the index circuit is more than the pump can handle.