All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
It is not clear to us from the question whether the marriage has been concluded or if you are still in the stage of engagement, but the marriage appears to have been concluded.
You should know that it is not permissible to accuse a Muslim of a disgraceful matter without evidence. The matter is even worse when accusing the husband. We have already discussed this in Fataawa 138540 and 117779, so please refer to them.
Also, if that man informed you about the sin which he had committed, then he is very wrong to have done so and he has to repent and conceal his sin. Abu Hurayrah narrated that he heard the Prophet say: “My entire nation is safe except those who sin openly or disclose their sins to the people. For example, one commits a sin by night and though Allaah hid it from the people, he comes in the morning, and says, "O so-and-so, I did such-and-such a deed last night," though he spent his night covered by his Lord (none knowing about his sin) and in the morning he removes Allaah's cover from himself.” [Al-Bukhari and Muslim]
Also, ‘Abdullaah ibn ‘Umar narrated that the Prophet said: “Avoid these filthy things which Allaah has forbidden, but if someone commits any of them, he should conceal himself with the screen of Allaah and repent, because if anyone uncovers his hidden sins (to us), we shall inflict on him the punishment prescribed by Allaah, the Al-Mighty.” [Al-Haakim]
If it is proven that he did so and he repented, then whoever sincerely repents, Allaah accepts his repentance. Then, you should not hold him accountable for what he did and you should live with him in a descent manner. He should also live with you in decency. Allaah says (what means): {And due to the wives is similar to what is expected of them, according to what is reasonable.} [Quran 2:228]
For more benefit on the rights between spouses, please refer to Fataawa 85308 and 83157.
It is not permissible for you to seek divorce as this is forbidden in Islam unless you have a valid reason. For information on the valid reasons for seeking divorce, please refer to Fatwa 131953. Even if we assume that there is a valid reason, it is not always in the wife's interest to seek divorce. Please refer to Fatwa 170305.
With regard to gaining weight, we think that this is a trivial matter which is easy to achieve, so you should try to do so; so the matter does not cause any disputes.
Finally, it should be pointed out that if you are still in the engagement stage, and you fear that such a dispute could affect the future of your marriage, then it would be better to break the engagement to avoid marriage taking place and then being vulnerable to divorce and problems. It is known that the engagement is only a promise and either party can terminate it whenever they wish.
Allaah Knows best.