Commentary for Eruvin 5:8

אנשי איש גדולה – that they (i.e., the residents of a large town) had a small town within the two-thousand [cubits] and they depart from their city and count and walk the path of the small [city/town] that is adjacent/nearby, they walk through all of the small town/city that is adjacent to them like four cubits and complete their measurement outside of it.

The people of a large town may walk through the whole of a small town, but the people of a small town may not walk through the whole of a large town.
How is this so? If a man was in a large town and deposited his eruv in a small town or if he stayed in a small town and deposited his eruv in a large town, he may walk through all the town and two thousand cubits beyond it.
Rabbi Akiva says: he only has the place of his eruv and another two thousand cubits.
The wording of our mishnah is somewhat difficult, and the talmudic sages already recognized different versions. However, the different versions are usually explained the same way. All agree that one who resides in a town and begins Shabbat there can travel two thousand cubits in all directions. However, the sages debate concerning a person who resides in one town but puts his eruv in another town. The question is: does this eruv, placed in another town, allow him to walk throughout the town and two thousand cubits around it, or is the area of the town included in the two thousand cubits? In other words, since he acquired his “Shabbat place” in that town through an eruv and not through his actually being there, is he treated as if he was there or do we look at this as a normal case of placing one’s eruv outside of one’s own town?
The other issue in the mishnah is concerning one who measures the two thousand cubits around his city and finds that another city is within this area. If the two thousand cubits ends in the middle of the city, all of the sages agree that he cannot go anywhere beyond the city on the other side. However, if the two thousand cubits ends at the end of the city, the city is treated as if it was merely four cubits in area and it doesn’t count for the two thousand cubit measure. Rather the two thousand cubits is completed on the other side of the city. This halakhah is the first section of the mishnah.
Section one: As I explained in the introduction, the wording of this mishnah is quite difficult. I shall explain the mishnah in the same that Albeck and Kehati do. The people of a large town, who have a small town entirely within their 2000 cubit Shabbat border, may walk throughout the entire small town and it counts only as four cubits toward their Shabbat limit. The remaining 2000 cubits surrounding the large town continue on the other side. For instance if there are 96 cubits from the large town to the small town, and the small town is 1800 cubits, he may walk through the whole small town and another 1900 cubits on the other side, since the town itself counts as only four cubits.
However, if the Shabbat border of a small town ends within a neighboring large town, the people from the small town can only walk within it up until their border ends.
We should note that according to this explanation, the size of the person’s original town does not seem to matter all that much. All that matters is whether or not the Shabbat border ends within the neighboring town.
Section two: The phrase “how is this so” does not seem to explain the previous clause in the mishnah. Hence, the Yerushalmi removes the word, whereas the Bavli adds in an entire clause. In any case, it seems that this is a new section and not a continuation of the previous one. This section deals with a person from one city who puts his eruv in another city. According to the first opinion, if one puts his eruv in another city that is within the 2000 cubits of one’s own city, the eruv makes the person as if he was a full resident of that city. Therefore, he may walk the full area of the city, plus 2000 cubits around it, just as can the residents of that city.
Rabbi Akiva disagrees and holds that the eruv in the other city works as do all normal eruvin they allow one to walk 2000 cubits beyond the eruv. If the city in which the eruv was placed goes further, he may not walk in that area of the city.

ואין אנשי עיר קטנה מהלכין את הגדולה – [the large town/city] in its entirety like four cubits, because the measurement of the [Sabbath] limits ended in the middle of the large city/town, therefore, the large city/town is not considered for them like four cubits and they don’t go there, but rather only until the end of their [Sabbath] limits.

כיצד מי שהיה מעיר גדולה – Our Mishnah is deficient and should be read as follows: The people of a large town/city walk through the entire small town/city but the people of a small town/city do not walk through all of the large city/town; in what case are these words said? When he measures two-thousand cubits. But, a person who places his Eruv within the city/town, whether the people of a large town/city placed it in a small town/city or whether the people of a small town/city placed it in a small town/city, they walk all the city/town that the Eruv is placed in it like four cubits. How so? He who was from a large city/town and placed his Eruv in a small town/city etc.

ור' עקיבא – disputes on the first Tanna/teacher and holds that the Eruv does not make the city/town in which it is placed like four cubits, and we don’t count two-thousand cubits other than from the place of the Eruv. But the Halakha is not according to Rabbi Akiva.