In a total world population of around 1.8 billion Muslims, radicals and terrorists are a minuscule minority. A recent article by CNN estimates the total number of members of Muslim terrorist groups as around .00625% of the world’s total Muslim population. Even if one assumes that there is a total number of Muslim terrorists several times that figure, to account for “lone wolf” extremists and currently unknown groups, one still finds only a very tiny percentage of Muslims involved in terrorism or extremist violence.
What is true is that Muslim terrorists are very much in the public eye, especially in the U.S. and Europe, to the extent that some people erroneously believe that extremist violence is unique to Muslims. There are several reasons for this:

Many actions of Muslim (and other) terrorists are deliberately designed to draw attention. The perpetrators of the 9/11 attacks knew, and intended, that images of these atrocities would dominate news around the world. Indeed, terrorism, which, on the scale practiced today, is something new in history, is intended precisely to draw public attention to the terrorists and their grievances.
Closely related to the foregoing fact is the reality that terrorist violence can and does strike Western countries and hence poses a real danger to their citizens; it is, therefore, inevitably a matter of legitimate concern to Western publics (although the chance of one’s being killed in a terrorist attack is about the same as being crushed by falling furniture).
There appears to be some clear media bias to emphasising terrorism and extremism from Muslims over that from other groups—even when terrorism from other sources poses a clear danger to people in the U.S. Until the murder of 49 patrons of a gay bar in Orlando, Florida this past June, the number of people killed by Muslim terrorists in the U.S. was actually slightly less than those killed by right-wing extremists who often identified themselves as Christians. Nonetheless, the Journal of Communication reports that 81% of domestic terrorism suspects are identified as Muslims in TV news, while the FBI reports only 6% of terrorism suspects are Muslim.
In other words, while there are only a small number of Muslim extremists and terrorists, especially in comparison to the world’s total Muslim population, they loom very large in the public mind—for reasons both legitimate and not.