The Sunni Muslims form the largest group (about 85%) of the Muslims. There are many brnches of smaller groups like Shias, Wahabis, Ahl-i-Hadith etc.
Islam is not the smallest it is the largest religion in the world.
Islam is the second largest religion: behind Christianity
Judaism, Christianity, and Islam (oldest to youngest). Islam, Judaism, & Christianity (largest to smallest).
Muhammad was not the founder of any branches of Islam. In fact, he was not even the founder of Islam, rather the messenger. As well as this, Muhammad and Islam discourage branches within Islam so it clashes with the question.
Islam has only two branches: Sunnah & Shieaa. The two branches are found in Iran with a Shiaa majority there.
The largest is Christianity, although that has several major branches : Roman rite, Greek rite, Protestant, Nestorian, Coptic and others. Islam is the second and it, too, has divisions, the most significant being Sunni and Shiite. Hinduism is third and it has a complex arrangement of beliefs. As for the smallest, that would have to be an autocreed, a belief that has but a single worshipper. An example, albeit a fictional one, could be the protagonist's religion in The Far Pavilions.
Itis neither the largest, second largest or smallest.
There are actually three branches of Islam that are dominant in at least one country in Southwest Asia. In order of popularity: Sunni Islam, Shiite Islam, Ibadi Islam.
By area, France is the largest and Malta is the smallest. By population, Germany is the largest and the smallest is Malta.
The two main branches of Islam in Iraq are Sunnis and Shiites. However, both branches share many common Islam principles and rules. Refer to related question below.
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By area, the largest is Alaska and the smallest is Rhode Island; by population, the largest is California and the smallest is Wyoming.