Hashem wants to spend time with us. The Ramban says that sefira is chal hamoad between Pesach and Shevuos. So Shevuos would also be Atzeres.
The Midrash says for Shemini Atzeres עשה לי סעודה קטנה make Me a small meal. Explains the Binah Leitim, Succos we sacrifice all the oxen against the umas haolom, for all other nations. When we have a big meal, we tend to be busy with the food but by a small meal the showing of comraderie and friendship becomes more important. Regarding the goyim, Hashem kavayachel is busy with food rather than show any love and friendship towards them but by the Jews, the sacrifice is small to be able to spend time of love with us.