סוכה כז.
"ט"ו ט"ו מגזירה שוה: מה להלן לילה הראשון חובה מכאן ואילך רשות אף כאן לילה הראשון חובה מכ"ו רשות"
Our Gemara makes a גזירה שוהfrom חמשה עשר that’s written by פסח , to חמשה עשר that’s written byסוכות . Just like by פסח there is a חיוב אכילת מצה only on the first night, after that אכילת מצה is only a רשות, one doesn’t need to eat מצה, so too by succah, there is a חיוב to eat in the סוכה only on the first night, after that if one chooses, he can only eat אכילת עראי & there is no חיוב to make a seuda & eat it in the succah. The [תרלט סי' ג'] שו"ע paskens this way that one must eat a כזית of לחם in the succah on the first night. The משנ"ב adds that even though this חיוב is derived from the above ג"ש from פסח & אכילת מצה only requires a כזית, it is still best to eat more than a כביצה in the succah to be חושש for those that hold the שיעור אכילה בסוכה is the amount that would require one to eat in the succah on the rest of the days of succos, which is יותר מכביצה.
In [ד"ה בליל] ביאור הלכה the חפץ חיים brings that many poskim wonder what the חידוש is that ליל יו"ט ראשון is a חיוב to eat in the succah. After all, on Yom Tov one is required to eat פת, so ממילא one will have to eat in the succah? Why the special drasha? He says he doesn’t understand this question. We know that if one is מצטער to eat on שבת ויו"ט he doesn’t have to, because the point is for עונג & for him it’s not pleasurable. But according to our לימוד this person would have to eat a כזית in the succah, because it’s a חיוב מצד מצות סוכה not a דין in עונג. So our drasha is very necessary! The גר"א holds that since by חג המצות it says שבעת ימים תאכלו מצות, this means that although the מצות עשה is specifically for the first night, one will still get a מצוה with every כזית of matzah that he eats all 7 days of Pesach. The משנ"ב says maybe according to the Gra since we compare succos to pesach, one should eat bread in the succah every day to be מקיים a mitzvah.
Rav Aharon Leib Shteinman zt”l was once asked [מרפסין איגרי דף קטו'], according to this shita of the Gra, that although there is no special מצות עשה to eat in the succah or to eat מצה on Pesach after the first night, but if one eats he is still מקיים a מצוה, why on the Yom Tov of Succos when we do eat bread in the succah even after the first night, we make a ברכת לישב בסוכה, yet on Chag HaPesach if we eat matzah after the first night we don’t make a special beracha even though we are being מקיים a מצוה? עיין בעל המאור סוף מס' פסחים.