grammar - Tenses - Differences in meaning of two sentences

I would like to understand the difference between two sentences and ask if they are gramatically correct.

1.) Sam gained weight because he had been overeating.

So, he ate much in the past and gained weight also in the past. It does not have nothing common with his present state. Maybe he lost his weight. Is it correct iterpretation?

2.) Sam has gained weight because he had been overeating.

He gained weight in the past, because he ate much also in the past. But he is obese now. The state of "being obese" continues from the past until now (we see an effect). Is it correct?

Thank you in advance.

Answer

Your first interpretation is exact.

For the second one the proper sentence should be: 'Sam has gained weight because he has been overeating'.
The Past Perfect Continuous Tense, which you used, must refer to the definite past moment (with the emphasis of duration).
But your second sentence refers to the present moment.
That's why the second sentence is grammatically wrong.