Allah says in the Quran that the wife has similar rights to what is against them according to what is reasonable. What I understood is that these are the rights which the man gets through obedience and the wife gets through kindness because she cannot command the husband. What I am asking is whether being unkind to one's wife is a major sin or a minor sin?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

The Islamic Shariah enjoined kindness to women and gentleness with them, as the Prophet said, "Be kind to women." [Al-Bukhaari and Muslim]

The Prophet also said, "The best of you are those who are the best to their wives, and I am the best of you to my wives." [At-Tirmithi]

On the other hand, the Prophet severely warned against injustice to the woman, as he said, "O Allah, I declare sinful any failure to safeguard the rights of two weak ones: the orphan and the woman." [Ibn Maajah]

If the husband's harshness with his wife involves injustice to her by abusing her or her property unlawfully, then in this case, it is a major sin.

Ath-Thahabi said, “The twenty-sixth major sin: injustice by consuming people's money and taking it unjustly, and wronging the people by beating them, insulting them, transgressing against them, and violating the weak ... It is also injustice that a man transgresses the right of a woman with regards to her dowry, spending on her, and clothing her." [Al-Kabaa’ir, p. 104]

As for the interpretation of the saying of Allah (which means): {…and due to the wives is similar to what is expected of them, according to what is reasonable. But the men have a degree over them [in responsibility and authority].} [Quran 2:228], then Abu Hayyaan said in Al-Bahr al-Muheet:

"The scholars differed in opinion about this similarity of what is expected from each one; it was said: similarity is in compliance and leniency with the other....it was also said: it is similarity in beautifying oneself for each other; Ibn ‘Abbaas said, 'I like to beautify myself for my wife as I like her to beautify herself for me, because Allah says: {And due to them [i.e. the wives] is similar to what is expected of them, according to what is reasonable.} [Quran 2:228].' It was also said: Similarity is in fearing Allah in treating their wives, in the same manner that the wives should fear Allah in treating their husbands... It was also said: Similarity means that the wives have the right upon their husbands to spend on them, have good marital relations with them, and not harm them, in the same manner that the wives should honor the command of their husbands and refrain from what their husbands forbid them from doing; so the similarity is in the obligation of what the husband does of this, and the obligation of the wife in honoring his orders and prohibitions, and not in the very nature of what is honored and done... It was also said: Similarity is general in all the rights of the husband upon his wife and the rights of the wife upon her husband."

Al-Baydhaawi said, "It means that they have rights on their husbands just as the husbands have rights on them in terms of the obligation of fulfilling those rights and entitlement to them, and not in the nature of the rights themselves. Men have a degree over them, which is an additional right and a virtue." [Tafseer Al-Baydhaawi, 1/141]

Besides, An-Nasafi said, "Similarity here is intended to mean similarity of the obligation in terms of it being a good deed, not the nature of the obligation itself; as it is not obligatory on the husband that if his wife washes his clothes or bakes for him that he does the like of that for her; instead, he does what is suitable for men to do."

Allah knows best.