A TXV acts as a fixed orifice above a certain temperature, usually 60 to 70 degF. Below that temp, it matches the flow based on bulb pressure (which follows evaporator exit temperature) vs evaporator pressure. Some valves are preset to add a fixed amount of superheat, where others have a superheat adjustment. The closer the charge in the bulb is to what's in your system, the better the txv will track and maintain a constant superheat.
The rated value on the valve is about 2/3 of the full-bore capacity of the valve. A 1/3 ton valve can open up to pass 1/2 ton of capacity. If you oversize the valve, it will overshoot its target and cause oscillation, or "hunting".
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