Making the second rak’ah long and the first rak’ah short

The Sunnah in prayer is to make the first rak’ah long and the second short, because of the report narrated by al-Bukhaari (759) and Muslim (685) from Abu Qataadah (may Allaah be pleased with him) who said: The Prophet (peace and blessings of Allaah be upon him) used to recite in the first two rak’ahs of Zuhr prayer the Opening of the Book (al-Faatihah) and two soorahs, making the first rak’ah long and the second short, and he made his recitation audible sometimes. In ‘Asr he used to recite the Opening of the Book and two soorahs, making the first rak’ah long. And he used to make the first rak’ah of Fajr prayer long and the second short. 

Al-Bukhaari (may Allaah have mercy on him) included this hadeeth in a chapter in his Saheeh entitled, “Chapter on making the first rak’ah long.” In this chapter he reported the hadeeth of Abu Qataadah which says that the Prophet (peace and blessings of Allaah be upon him) used to make the first rak’ah long in Zuhr prayer and the second rak’ah short, and he also did that in Fajr prayer. 

Al-Haafiz ibn Hajar said: The phrase  “Chapter on making the first rak’ah long” refers to all the prayers, which is apparent from the hadeeth mentioned. End quote from Fath al-Baari, 2/305 

Al-Nawawi (may Allaah have mercy on him) said: The phrase “he used to make the first rak’ah long in Zuhr prayer and the second rak’ah short” is something concerning which the scholars differed with regard to following the apparent meaning. There are two views among our companions, the more well known of which is that it should not be made lengthy, and the hadeeth is to be understood as meaning that it was made long because of the opening du’aa’ and prayers for refuge with Allaah, or because he sensed that someone was about to join the prayer, and so on, not because of recitation. The second view is that it is mustahabb to make the recitation long in the first rak’ah on purpose. This is the correct view which is in accordance with the Sunnah. End quote from Sharh Muslim, 4/175 

Based on this, if the imam makes the second rak’ah longer than the first, and he always does that and makes it a habit, then this is contrary to the Sunnah, and he should be advised to follow the Sunnah of the Prophet (peace and blessings of Allaah be upon him), for the best guidance is the guidance of Muhammad (peace and blessings of Allaah be upon him). But making the first rak’ah long than the second is not obligatory and is not an essential condition of the prayer being valid, rather it is preferable.