The restaurant is full of people.
Perhaps you will suddenly wonder, why is "で" used here?
If it were me, I would think "人はいっぱいです", this short sentence is correct.
We know that there cannot be two "は" in a sentence. But from the conclusion, there can be two "は", especially and only when comparing:
- 私は今日は行けない。 I can't go today (I mean, I can go another day)
The first "は" is the subject of the prompt, and the second "は" is the comparison.
Therefore, "レストランは人はいっぱいです" may be feasible.
So, why use "で"?
According to my previous blog, I found this
5. Show reason
- 病気で休む。Rest due to illness.
- 受験（じゅけん）準備で忙しい。Because preparing for the exam,I am very busy.
I got it.
Therefore, this sentence should be translated into:
"The restaurant is full because of people."
At the same time, I think not only "because of", I think "made of" may also be possible, maybe other...Anyway, it will not be "comparison".
- 鍋の中は豆でいっぱいだ。 The pot is full of beans.