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.
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.
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.
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.
He always thinks the grass is greener on the other side.
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.
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.
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.
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.
You say 'the biggest fish is always the one that got away,' but that's an English proverb.
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.
That's just wishing for what you don't have.
They say too much is as bad as too little, so you shouldn't regret it too much.
What's done is done; let's forget about that matter.