Why fight the Midianites with such a huge army?

Question

G-d promises the Israeli Nation that (Lev 26,6):

וְרָדְפוּ מִכֶּם חֲמִשָּׁה מֵאָה, וּמֵאָה מִכֶּם רְבָבָה יִרְדֹּפוּ
וְנָפְלוּ אֹיְבֵיכֶם לִפְנֵיכֶם לֶחָרֶב׃

Five of you shall give chase to a hundred, and a hundred of you shall give chase to ten thousand;
your enemies shall fall before you by the sword.

According to the number of asses or little girls captured I estimate the number of Midianites to ca. 50,000 (there were 60,000 asses and 30,0000 girls, roughly one per family).

That would require 50,000/100 = 500, so only around 500 Israelites were needed to wipe them out completely. As all the sinners with Baal peor were already killed I presume that the rest were pious and the Blessing should have been fulfilled.

Why then, did Moses require 12,000 Israelites in Bamidbar 31,4-5 to wage war on the Midianites?

Answer 1

The pasuk can be taken as literature parse and shouldn't be taken literally, just like:

Lev 26, 37

וְהַנִּשְׁאָרִ֣ים בָּכֶ֔ם וְהֵבֵ֤אתִי מֹ֙רֶךְ֙ בִּלְבָבָ֔ם בְּאַרְצֹ֖ת אֹיְבֵיהֶ֑ם וְרָדַ֣ף אֹתָ֗ם ק֚וֹל עָלֶ֣ה נִדָּ֔ף וְנָס֧וּ מְנֻֽסַת־חֶ֛רֶב וְנָפְל֖וּ וְאֵ֥ין רֹדֵֽף׃

As for those of you who survive, I will cast a faintness into their hearts in the land of their enemies. The sound of a driven leaf shall put them to flight. Fleeing as though from the sword, they shall fall though none pursues.

No one runs away from sound of leaves...

If you insist of taken it literally, so as @mbloch said, people of Israel should be in a condition of

אִם־בְּחֻקֹּתַ֖י תֵּלֵ֑כוּ וְאֶת־מִצְוֺתַ֣י תִּשְׁמְר֔וּ וַעֲשִׂיתֶ֖ם אֹתָֽם׃

Which is unlikley right after the sin of Moav.

Moreover, winning a war like this is a miracle, and one shouldn't trust a miracle to happen (ירושלמי שקלים ו ג, Yerushalmi Shkalim p6 Halach 3, Gemara)

Answer 2

I know you are asking about 100 fold damage, But The Gemora in Moed Katan 16b answers a question regarding why David could not cause 1000 fold damage mentioned in Devarim 32,30:

על שמונה מאות חלל בפעם אחת שהיה זורק חץ ומפיל שמונה מאות חלל בפעם אחת והיה מתאנח על מאתים דכתיב (דברים לב, ל) איכה ירדף אחד אלף
The expression: “Against eight hundred people, which he slew at one time,” means that he would throw an arrow in the air and with it kill eight hundred people at one time. And David would sigh over the two hundred who were missing from fulfilment of the Torah’s promise, as it is written: “How should one man chase a thousand” (Deuteronomy 32:30).

יצתה בת קול ואמרה (מלכים א טו, ה) רק בדבר אוריה החתי
A Divine Voice issued forth and said by way of explanation as to why the promise was not entirely fulfilled: “Because David did that which was right in the eyes of the Lord, and turned not aside from anything that He commanded him all the days of his life, save only the matter of Uriah the Hittite” (I Kings 15:5). Had David not committed this sin, then all of the promises mentioned in the Torah would have been fulfilled in their entirety through him.

Even David Hamelech who was extremely pious, was not great enough to kill 1000 people in one shot so it seems that only someone absolutely perfect would be able to fulfil the Passuk in its entirety. Gemora Avoda zara 4a says, that David did not have Bechira when he sinned with Batsheva (Libi chalal bekirbi means David ousted his Yetzer Hora like the Avos) rather it was orchestrated by Hashem, and he still couldn't kill 1000 people like the passuk states.
Those who who fought in Midian, might have been less great than David and not merit to kill so many people as well. Even though they did not sin with Baal Peor, they needed to atone for looking at the immodest women as Gemora Shabbos 64a-b says their donation (Bamidbar 31,50)of golden jewelery including Agil which went on the breast and Cumaz which went on the womb were meant to atone for sinning by looking at the women, which is maybe why less people killed per each person.