Who has ever heard of towns with walls that reach the sky?
Rashi (citing Chulin 90b): The Torah is speaking in exaggerated terms.
QUESTIONS ON RASHI
Rashi writes (from Chulin 90b) that "uVetzuros ba'Shamayim" teaches that the Torah exaggerates. Here it cites what the people heard from the spies. Is it not more reasonable that the Gemara learns from (9:1), where the Torah itself says so?!
Pesikta Zutresa and Yalkut Shimoni, both citing R. Shimon ben Gamliel, learn this from our verse. Daf Al ha'Daf (Chulin 90b) - Pardes Yosef ha'Chadash Balak 129) says that we may learn from words of Resha'im recorded in the Torah, e.g. "Lo Ya'aseh Ken bi'Mkomeinu" (Bereishis 29:26), "b'Eleh Tenagach Es Aram" (Melachim 1:22:11). According to the Gra's Hagahah of Sifri, R. Shimon ben Gamliel learns from our verse and 9:1. 1
Perhaps this is because the Torah recorded also Sheker of the spies, e.g. "Lo Nuchal La'alos