What is the meaning of "v'Nakel Zos"?
Rashi: This miracle is light and small in Hashem's eyes, and He will add another miracle, and give Mo'av in your hands.
Radak (citing Tanchuma Pinchas 3): When Elisha commanded to topple the trees (verse 19), they asked, it says "Lo Sashchis Es Etzah" (Devarim 20:19)! He answered, that refers to other nations. Mo'av is disgraced in front of Me - "v'Nakel Zos}, "Lo Sidrosh Shelomam v'Tovasam" (Devarim 23:7) refers to the good trees among them.
Malbim: The miracle that Hashem does for a nation or a person, it must come first and primarily for him, and not second, and by chance. Since the water is for Yisrael, it needed to fall here (in the Midbar), where it is needed, and not in Eretz Edom. It should not seem that the water was primarily for Edom, and Yisrael received it by chance, like "Ki Etzak Mayim Al Tzamei v'Nozlim Al Yabashah" (Yeshayah 44:3). Do not say that it is a bigger miracle, for they will not see wind or rain, and even so, the valley will fill with water. Just the contrary! It would be a bigger miracle if rain fell here, where it does not fall via nature; it was a dry desert. Rain is normal in Eretz Edom, and its rivers often flow outside. This is a small miracle, close to nature.
Is conquering Mo'av connected to the smallness of the miracle?
Malbim: Yes. (a) The purpose of this miracle (giving water) is in order to defeat Mo'av. The water is only a means. Therefore, Hashem is not concerned that it be a big miracle far from nature (refer to 3:18:1:2). (b) Yisrael were not worthy of two miracle at once (rain and defeating Mo'av). Had the rain come via a big miracle, it would have reduced their strength, and they would not conquer Mo'av. Therefore, Hashem chose that the water come easily, close to naturally.
