Sukkah says Psul because "Nifishi Milay" and the Mishna would have to explain too much. Mavoy is not so halachically complex, so there it can afford to provide alternative suggestions. That is the pashut understanding.
But Rashi is mashmah that Mavoy is also complex. only that it is "Lo Nafish Milay - Boso Mishna". If so, why doesn't the Tannah also seperate the hilchos Sukkah into three (or two) different mishnayos and teach a takantah by each?!
Yaakov Siegel, Miami, Florida
It seems to me that the Pshat is as follows. The first Mishnah in Sukah and the first Mishnah in Eruvin both discuss a Sukah or a Mavoy which were made wrongly and for which the way of correcting is stated. However there are 4 mistakes mentioned for Sukah: 1. Too high 2. Too low 3. Not enough walls 4. Not enough shade. Each of these problems has a different solution.
If one looks at MISHNAH ERUVIN 2a one observes that there are 2 mistakes: 1. Too high 2. Too wide. However the point is that only one remedy is necessary there- "Yema'et", make it less. The key lies in RASHI here DH SUKAH that the "Takanah" for the 4 problems of Sukah is different in each case. "Nifishi Milay" is referring to the amount of Takanos, not the amount of Halachos. In contrast there is only one Takano for both of the problems in the Mishnah in Eruvin. When Rashi here DH MAVOY writes that there are not many things "in that Mishnah", he is not intending to suggest that there are many Halachos of Eruvin elsewhere, but rather that there is only one sort of amendment necessary in the Mishnah for a wrongly built Mavoy.
Therefore, it would not help for the Tana to separate the Halachos of Sukah into 4 different Mishnayos because it would still have to teach 4 different Takanos. Consequently, our Mishnah here said "Pesulah" to shorten the Mishnah by not mentioning all 4. In contrast the Takanos for a faulty Mavoy can be concisely specified because there is only one!
KESIVAH U'CHASIMAH TOVAH
D. Bloom
I found, BS'D, that RITVA and MEIRI explain the Gemara in a similar way to what I wrote.
Ritva writes that there are certainly very many Halachos of Mavoy. However the Gemara means that in our Mishnah there are many things for which, if the Takanta would have been taught this would have made the Mishnah much longer. It would have had to write that 1. a Sukah that was too tall must be lowered. 2. a Sukah under 10 Tefachim must be raised . 3. three walls must be made for a Sukah with presently less than three .4. Sechach must be added to a Sukah with more sun than shade.
In contrast, only one Takanta is necessary for Mavoy so this does not make the Mishnah there any longer. Ritva writes that his explanation follows Rashi. He writes that the rule is that if one can write a Takanta concisely this is preferable, but brevity is more important than teaching a Takanta.