Rashi on Numbers 13:18/רש"י על במדבר י״ג:י״ח

1את הארץ מה הוא AND SEE THE LAND; WHAT IT IS — There are countries which rear strong people and there are countries which rear weak people; there are such as produce a large population and there are such as produce a small population (Midrash Tanchuma, Sh'lach 6).

2החזק הוא הרפה WHETHER THEY ARE STRONG OR WEAK — He gave them a sign: if they live in open cities they are strong, since they evidently rely on their own strength, but if they live in fortified cities they are weak (Midrash Tanchuma, Sh'lach 6).