Translation guide
This English proverb means that people tend to exaggerate the size or importance of something they failed to obtain, especially in fishing or missed opportunities. In Japanese, there is no direct equivalent proverb, but the idea can be expressed through phrases about exaggeration, regret, or the tendency to romanticize the past.
To express that someone is making a lost opportunity seem better than it actually was
A direct translation that is sometimes used to convey the English proverb. It is understandable but not a traditional Japanese saying.
彼はいつも「逃した魚は大きい」と言っている。
He always says 'the biggest fish is always the one that got away.'
A slight variation of the above, using 逃がした instead of 逃した. Both are used.
逃がした魚は大きいって本当だね。
It's true that the one that got away is always the biggest.
More specific to fishing, literally 'the fish you failed to catch is big'. Less common as a general proverb.
釣り逃した魚は大きいと言うけど、本当にそう思うよ。
They say the fish you fail to catch is big, and I really think so.
To comment on how people often think what they don't have is better
Literally 'the neighbor's lawn is green', equivalent to 'the grass is always greener on the other side'. It captures the idea of envying what you don't have, similar to the fish proverb.
彼はいつも隣の芝生は青いと思っている。
He always thinks the grass is greener on the other side.
A noun phrase meaning 'asking for what one doesn't have' or 'the grass is always greener'. It describes the tendency to desire what is out of reach.
それはないものねだりだよ。
That's just wishing for what you don't have.
A proverb meaning 'too much is as bad as too little', but can be used to reflect on overvaluing something missed. It's more about moderation, but contextually can apply.
過ぎたるはなお及ばざるがごとしと言うから、あまり悔やまないほうがいい。
They say too much is as bad as too little, so you shouldn't regret it too much.
To convey the feeling of regret when an opportunity is lost
A proverb meaning 'repentance comes too late' or 'it's no use crying over spilt milk'. It fits the regret aspect of the fish proverb.
後悔先に立たずだから、次に生かそう。
It's no use crying over spilt milk, so let's learn from it for next time.
Literally 'spilt water doesn't return to the tray', meaning what's done cannot be undone. It emphasizes the irreversibility of a missed opportunity.
覆水盆に返らず、もうあの話は忘れよう。
What's done is done; let's forget about that matter.
There is no traditional Japanese proverb that perfectly matches 'the biggest fish is always the one that got away'. The direct translation 逃した魚は大きい is sometimes used but may sound like a literal translation of the English. For natural Japanese, consider using a more common proverb like 隣の芝生は青い or 後悔先に立たず depending on the nuance.
逃した魚は大きいと言うけど、それは英語のことわざだね。
You say 'the biggest fish is always the one that got away,' but that's an English proverb.