Sukkah 20A

והכי קאמר: מחצלת הקנים גדולה, עשאה לשכיבה ⁠— מקבלת טומאה, ואין מסככין בה. טעמא דעשאה לשכיבה, הא סתמא ⁠— נעשה כמי שעשאה לסיכוך, מסככין בה. (קטנה, עשאה לסיכוך ⁠— מסככין בה. טעמא דעשאה לסיכוך, הא סתמא ⁠— נעשה כמי שעשאה לשכיבה, ואין מסככין בה). ואתא רבי אליעזר למימר: אחת קטנה ואחת גדולה, סתמא ⁠— כשרה לסיכוך.

And this is what the Mishnah is saying: With regard to a large mat of reeds, if one produced it for the purpose of lying upon it, it is susceptible to ritual impurity, and one may not roof a sukka with it. The reason is that one produced it specifically for the purpose of lying upon it; however, by inference, a mat that one produced without designation becomes as a mat produced for roofing, and one may roof a sukka with it. With regard to a small mat of reeds, if one produced it for roofing, one may roof a sukka with it. The reason is that one produced it specifically for roofing; however, by inference, a mat that one produced without designation becomes as a mat produced for the purpose of lying upon it, and one may not roof a sukka with it. And R' Eliezer comes to say that both a small mat and a large one produced without designation are fit for roofing.

אמר ליה אביי: אי הכי, רבי אליעזר אומר: אחת קטנה ואחת גדולה, ״אחת גדולה ואחת קטנה״ מיבעי ליה!

Abaye said to him: If so, if their dispute is only with regard to a small mat, then instead of saying: R' Eliezer says: Both a small mat and a large mat, the Mishnah needed to say: Both a large mat and a small mat. In a phrase with the format: Both this and that, one typically mentions the more obvious item first. Why then, does R' Eliezer mention the small mat first, if it is with regard to the small mat that they disagree?