The current issue of the New Yorker reports that, according to Paris-based Iranian sociologist Farhad Khosrokhavar, "of France's 64,000 prisoners, up to 60% are Muslim. (Muslims are thought to compose only 8% of the population.)"
Based on data from 2011, Pew Research Center estimated that Muslims made up 9% of the 1,598,780 state and federal prisoners in the United States. Pew also reported that as of 2010, about 0.8% of the U.S. population was Muslim, up from 0.6% in 1990. Data allegedly provided by the federal Bureau of Prisons reported that, as of 1997, Muslims made up 7.27% of the federal prison population.
In other words, Muslims are overrepresented in the French prison population by 7.5 times their percentage of the population. In the United States, although their overall percentage is smaller, Muslims are overrepresented in the prison population by 11.25 times their percentage of the population.
Why? A couple of possible explanations spring to mind, neither one of which is exclusive.
One possibility is that Muslims may be committing crimes at a higher rate than other groups.
Another is that large numbers of prisoners are converting to Islam. There is evidence that this is in fact the case. The New Yorker article offers a sociological explanation of why and how this happens in French prisons. Conversion also appears to be common in U.S. prisons. See also here, here, here, and here.
What makes this truly disturbing is the form of Islam to which prisoners are converting. Much has been written about the radicalization of Muslims in French prisons (for example, here, here, here, and here). It is also happening in U.S. prisons (see here and here). Despite earlier efforts to obscure this issue, it isn't going away any time soon. Expect it to get worse before it gets better.