I am not sure why. But I can tell you how to fix it.
The default settings for the Arabic alphabet on most operating systems and programmes are just weird. I recently wrote that reading Arabic (newspapers, books, etc.) on the Internet can be irritating. Many people told me that they would love to read more Arabic on the Internet but they get tired after some time. The Arabic script, for some reason, is always smaller compared to Latin characters. Furthermore, the font is sometimes weird and might even produce mistakes as not all characters are correct.
A couple of weeks ago I recommended an extension called Huruf (حُرُوف) which does a great job and works perfectly fine with Google Chrome and Firefox. Some readers were asking me to tell them again were they can download it
The extension was written by Jackson Petty. You can get it here for free: https://github.com/jopetty/Huruf
Huruf makes reading on-screen Arabic easier by increasing the font size of all characters within the unicode ranges of the Arabic script; this means that Huruf will work on any language that uses the Arabic script, not just Arabic (e.g. Persian, Punjabi, Urdu, etc). You can set the font size and line height to be up to 150% of the default value.
Furthermore you can choose your preferred Arabic font. By default it is Droid Arabic Naskh which is one of the best free fonts available. This type of font is is optimized for reading Arabic script on screens. The designer paid attention that the large “loop height” and “tooth height” help prevent readers from having to zoom web pages to a larger size in order to read them. The traditional Naskh forms are softened for less formal documents such as periodicals and journals.
Note 1: If you use other browsers, such as Safari or Explorer, or if you encounter problems with the extension Huruf, Alex Strick van Linschoten, a passionate Arabic and Farsi language lover, offers some approaches to solve this problem on his blog.
Note 2: In an article published on wired.com the author explains why typeface desing has a western-normativity problem. The simple answer is that most Arabic typefaces have been designed by Latin-langauge typographers.