Electrical Braking: Why does voltage decrease with a decreased rheostatic load?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Dynamic braking circuit.

Don't forget that your motor has internal series resistance. As R2 approaches a short-circuit more and more of the power will be dissipated in the internal resistance of the motor.

Try measuring the motor terminal resistance and then do a voltage divider calculation using that number and your external resistor. Hopefully the maths will make sense of the situation.