Learn Turkish: Yediğimiz porsiyon çok küçüktü, bu yüzden puanım ortalama. - The portion that we ate was very small, therefore my rating is average.

The root is ye- (to eat). The suffix -dik makes it an object relative participle (meaning "that" or "which"). Then we add the possessive -imiz (our). Because -imiz starts with a vowel, the k in -dik softens to ğ. So it acts like an adjective literally meaning "our eaten".

Why is the past tense ending on küçüktü spelled with a "t" instead of a "d"?

The past tense suffix for "was" is normally -di. However, because küçük (small) ends in a hard consonant (k), consonant harmony changes the d to a t. Vowel harmony also changes the i to ü to match the last vowel of the word.

What does bu yüzden literally translate to?

Bu means "this" and yüzden means "from the face" (root yüz + "from" suffix -den). Put together, bu yüzden is a very common fixed phrase that simply means "therefore" or "because of this".

Where is the verb in the second half of the sentence: puanım ortalama?

There isn't a visible verb word! In Turkish, present tense "to be" verbs (am/is/are) are often just implied when describing something in the third person. Puanım ortalama simply means "my rating [is] average".