You eat like a bird. は食を細いことを表す慣用句です。
like a bird = 鳥のように
逆にたくさん食べる人のことを You eat like a horse.と言います。
like a horse = 馬のように
You eat very little. は「とても少ない量をたべるのですね」という
意味です。
例文
She eats like a bird. She always can't finish her plate.
彼女はとても小食だ。いつも食事を食べきれないんだよ。
All of these sentences show that the person eats small amounts or little. You could also just say 'you don't eat much' without the word 'very' and it would still make sense.
Having a small appetite indicates that someone doesn't feel as hungry as some other people and therefore eats less. You can also have the opposite of this and say 'you have a big/large appetite'. This would refer to someone who eats a lot or more than the people they are with.
Much and a lot are words to show a higher amount, but by putting the negative part of the sentence with them, they now mean less, as in the examples above. So using 'you don't eat...' changes the meaning of the words.
これらどの文も、その人の食べる量が少ないことを表します。
'very' を加えず、単に 'You don't eat much'（少食ですね）と言っても伝わります。
'Having a small appetite'（少食である）は、その人が他の人ほど食欲がなく食べる量が少ないことを表します。
その反対は、'You have a big/large appetite' です。これはたくさん食べる人あるいは周りの人よりも多く食べる人を指します。
'Much' と 'A lot' は量が多いこと表しますが、否定語とともに用いると上記の例のように「少ない」という意味になります。ですから、'You don't eat...' を使うことで、これらの単語の意味が変化します。
Small appetite は 少食、という意味です。
You have a small appetite.
(あなた少食なんだね)。
You have a small appetite today. Did you eat something before dinner? (今日あなた少食ね。晩御飯の前に何か他に食べた？)
参考に！
"light meal" も言えると思います。食事の量が少ない場合、"light meal" と言います。
例文：
You've recently been having a lot of light meals. Are you on a diet? 「最近少食が多いですね。ダイエット中ですか？」
No, I always have light meals, not just recently. 「いいえ、最近じゃなくて、いつも少食だよ」
"You don't eat much, do you!"
"How come you eat very little."
In this first expression "You don't eat much, do you!" you are expressing concern for the person. It communicates to the person that you are a bit surprised and that you would like to know the reason as to why they eat so little. It is a formal way of asking. In the second expression you are asking the same but its really informal, just flat out asking about something you noticed. Use this last expression mainly with people you know and not strangers or co workers.
"You don't eat much, do you!"（小食ですね）
"How come you eat very little."（何でそんなに少ししか食べないの）
一つ目の "You don't eat much, do you!" では、相手を気遣っています。軽い驚きとともに、その理由が気になっていることも表します。これはフォーマルな言い方です。
二つ目の例も同じことを尋ねていますが、こちらはすごくインフォーマルです。気付いたことを率直に尋ねているという感じです。この二つ目の表現は、知らない人や同僚には使わず、主に知り合いに使うようにしましょう。
"Seems like" is a way of saying that you are under the impression of whatever you believe to be observing, as in this case, the appearance of a small appetite. A small appetite means that the person wants to eat a very little portion of food.
Another word that is used to imply how much someone eats is "eater," for example, "He is a big eater" or "He is a small eater." These are examples of more informal expressions.
To imply that someone has a small stomach means that he/she does not have much room for food in his/her stomach. To say this is another way to tell him/her that he/she does not seem to eat very much.
"Seems like" は「私には～のように見える」という意味です。この場合は、"Small appetite" に見えるということです。"Small appetite" は「食欲があまりない」という意味です。
人の食べる量について伝えるときには、"eater" も使えます。
例えば：
"He is a big eater"（彼は大食いです）
"He is a small eater."（彼は小食です）
これらはインフォーマルな表現です。
"have a small stomach" は「胃袋が小さい」という意味です。これでも「あなたは小食ですね」と伝えることができます。
If you notice that someone doesn't eat a lot, this is a polite way to open up the topic. Some people may have an eating disorder or feel uncomfortable eating a lot in front of others so it is important to be polite and understanding about the topic and not force it if the person seems to be uncomfortable. I hope that this helps. :)
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