Eruvin 61A

ואין אנשי עיר קטנה מהלכין את כל עיר גדולה.

And the residents of a small city may not walk through an entire large city.

מאי טעמא ⁠— לאו משום דהני כלתה מדתן בחצי העיר, והני כלתה מדתן בסוף העיר.

What is the reason for this difference? Is it not because these, the residents of the small city, their measure of 2,000 cubits terminated in the middle of the large city, and therefore they may walk only to the end of their 2,000 cubits; and these, the residents of the large city, their measure of 2,000 cubits terminated at the far end of the small city, allowing them to walk through the entire city as though it were 4 cubits and complete the 2,000 cubit measure of their Shabbat limit on the other side of the city?

ורבי אידי, ״אנשי אנשי״ תני, ומוקים לה בנותן. אבל מודד לא תנן.

And R' Idi, who said that R' Yehoshua ben Levi’s statement has no source, may hold that the Mishnah teaches the two cases with the same formulation. Just as it states: The residents of a large city may walk through an entire small city, it similarly states: The residents of a small city may walk through an entire large city. His version of the Mishnah did not state that the residents of a small city may not walk through an entire large city. And he establishes the Mishnah as referring to one who placed his eiruv inside the other city. Consequently, that city becomes his Shabbat residence, and he may walk anywhere in that city and an additional 2,000 cubits beyond it. But we did not learn anything about one who was measuring 2,000 cubits from his Shabbat residence outside the city, in which case it makes a difference whether the entire city is within his 2,000 cubits or whether only part of it is within this limit.