All these sentences explan that you ate a lot more when you lived in Japan.
I eat more when I am in Japan!
I used to eat more when I lived in Japan!
My appetite was much better in Japan!
"appetite" - a hunger for food
これらの文はどれも「日本にいたときの方がずっとたくさん食べていた」と伝えます。
I eat more when I am in Japan!（日本にいるときの方がたくさん食べます）
I used to eat more when I lived in Japan!（日本にいたときの方がたくさん食べていました）
My appetite was much better in Japan!（日本にいたときの方がずっとたくさん食べていました）
"appetite" - 食欲
今より = than (I do) now.
日本にいた時 = when I was in Japan
の方が= rather than
たくさん食べていた = I ate a lot
ときを強調したいなら、「now」と過去形を使えます。
I eat less now than I did in Japan.
I ate more in Japan than I do here (in Canada) now.
"I actually ate a lot more in Japan"は
「実は日本にいたときの方がたくさん食べていた」
"Actually"の後に相手が驚くことを言います。ホストファミリーがカナダの方で食べていると思ってる可能性が高いので、驚くでしょう！
2つ目の英文は
「ここより日本にいたときの方が食べていた」との意味になります。
1つ目の方が自然でおすすめです。是非使ってみてください！
I ate more food when I lived in Japan than I do now.
I ate more food when living in Japan than I do here in Canada.
The phrase "more than" is a good way to compare two things. It can be used for food or other nouns such as money (I make more money than my brother). Make sure to put the noun in between "more" and "than."
"more than" は二つのものを比較するときのフレーズです。
"food" 以外の名詞も使うことができます。例えば、"money"。
I make more money than my brother（私は弟よりも稼いでいます）
名詞は "more" と "than" の間に入れてくださいね。
I don't eat as much here as I did in Japan. / I had a bigger appetite in Japan than I do here. - These sentences can be used to express that you ate more food in Japan than you do where you are currently at.
I don't eat as much here as I did in Japan.（日本にいるときほどは食べません）
I had a bigger appetite in Japan than I do here.（日本にいるときの方がよく食べていました）
これらの文では「今よりも日本にいたときの方がよく食べていた」と伝えています。
When in a different country the climate/weather or food might be different so you might eat more or less than in the country you are used to so you explain this by saying 'I ate more when I lived in Japan' or 'I don't eat as much here as I do in Japan'
外国に行くと気候や食べ物の違いから食べる量が増えたり減ったりすることがあります。
これは次のように説明できます。
'I ate more when I lived in Japan'（日本にいるときはもっと食べていました）
'I don't eat as much here as I do in Japan'（日本にいるときほどは食べていません）
When I was back in Japan, my appetite was considerable greater than it is here
Since arriving here from Japan, my appetite has somewhat diminished
I eat less here than I did in Japan
You are studying abroad in Canada. You want to explain that you used to eat more when you lived in Japan (than you do now). Above are a few suggestions for that case.
You could always say, "Japanese food is better than Canadian food, so I tend to eat more when I am in Japan." But maybe you don't want to often anyone. So it's better to say something simple like, "In Japan I eat more" or , in the past, "When I was living in Japan, I ate had more appetite" maybe because you are/were used to Japanese food, and you are not so much used to Canadian food, yet.
"In comparison to now..." or "Unlike now.." are good ways to start the sentence.
"Japanese food is better than Canadian food, so I tend to eat more when I am in Japan."（日本の食べ物はカナダの食べ物よりおいしいので、日本にいるときの方がよく食べます）
と言うこともできます。
ただ、相手を不快な気持ちにはさせたくないですから、例えば
「（まだカナダの食べ物に慣れていないので）日本にいるときの方がよく食べます／日本に住んでいるときはもっと食べていました」
のような言い方の方が無難です。
"In comparison to now..."（今と比べて）または "Unlike now.."（今と違って）で文を始めることもできます。
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