The answer is in the following Gemara: When more is not better, there is no need to discuss a quantity, minimum or maximum; it is irrelevant.
The Mishna says:
These are the commandments for which no definite quantity is prescribed [no minimum or maximum]:
-Peah [leaving some produce for the poor during harvest],
-Bikkurim [first fruit brought to the Temple],
-Visits [to the Temple on the festivals],
-Performing acts of kindness, and
[Also] Honoring father and mother…
-And bringing peace between a man and his fellow. [Peah 1:1]
The Talmud says:
Rabbi Berechyah asked: There are many others. Why are they not mentioned? [For example:]
-The Torah says: For the ceremony of Sotah, the wife suspected of adultery,
The kohen shall take... dust from the floor of the Tabernacle and put it into the [bitter] water. [Num. 5:17]
How much dust must be placed in the water?
-The Torah says: The ashes of the red heifer must be mixed with spring water. [Num. 19:17] How much of the ashes is required?
-The Torah says: If a man dies childless, his widow must marry his brother. If he refuses, he must submit to chalitzah: The woman takes off the man’s shoe and spits on the ground before him. [Deut. 25:9] How much spittle is required?
-The Torah says: To purify someone afflicted with leprosy, the kohen kills a bird and puts some of its blood in water, then sprinkles it on the diseased person seven times. [Lev. 14:5] How much blood is required?
The rabbis answered: The Mishna lists only those commandments for which more is better -- increasing the amount increases the merit. Yours are not in that category. [Peah Y 2b]
This answers the question. More water is not needed because more water is not better.