We didn't order dessert because the main course portions were already very filling.

portions 是 portion 的复数形式，portion 的意思是"部分、份量"，在这里特指"食物的分量"。 were filling 中的 filling 是形容词，来自动词 fill（填满），表示"能让人吃饱的、有饱腹感的"。