Ramban's commentary on Bamidbar 31:6 says:
וישלח אתם משה אלף למטה -
לא שלח שם כל עם הצבא, ואע"פ שהיו המדיינים עם רב והערים בצורות גדולות מאד.
והטעם כי הנכשלים בבנות מואב היו רבים ואינם ראויים לנקמת ה', על כן בחרו אנשים צדיקים וידועים לשבטיהם.
ויתכן עוד שנאמר, כי משה נצטווה צרור את המדיינים (לעיל כה יז): ונקום נקמת בני ישראל (לפנינו פסוק ב), ושלח שם מועטים להכות בערי הפריזי וכל עץ טוב להפיל וכל מעין מים לסתום וכל חלקה טובה להכאיב באבנים כמנהג השוללים, ולא ציווה להם דבר רק להינקם כאשר תמצא ידם, והש"י אשר לו המלחמות נתן מדין ומלכיהם ועריהם בידם. ועל כן קצף על הנשים היודעות משכב זכר לא דבר אחר, וציוה בטף הזכרים לנקמה:
In my translation:
And Moses sent them, a thousand of every tribe - He did not send all who were able to go to war, even though the Midianites were numerous and their cities very large and fortified.
And the reason is because those who failed through daughters of Moab were numerous and not fit for God's vengeance, so men who were righteous and well known [from] among their tribes were chosen.
It is also possible to say, that Mosheh was commanded "harass the Midianites" (25:17), and "avenge the children of Israel" (31:2), and sent a small number to attack the unwalled towns and take down every good tree and seal every water fountain and damage with stones every good plot of land as is the custom of plunderers, and only commanded them to avenge as much as they were able to, but the Lord, blessed be He, who to Him are all wars, delivered Midian, their kings and their cities into their hands. That is why he was angry over keeping the women alive but over nothing else, and commanded regarding the under-aged males for vengeance.
In short, according to Ramban, the Midianites were numerous and well fortified in their cities, and sending a larger army to fight them would have been expected. He suggests two possibilities for the small number sent:
- Only those who hadn't succumbed to the daughters of Moab were worthy of taking part in God's vengeance, and they were few.
- It was only meant to be a retaliation, and of a limited scale, so fewer men were needed. By God's graces, they achieved far more than they set out to do.