Why do different aircraft have different maximum operating temperatures? I was reading this Wired article about the recent Phoenix heat grounding some jets. It's a great read on why heat is detrimental to lift, but it raises a point it never answers (emphasis mine)

Phoenix just provided another reason to hate flying: the heat. With temperatures there expected to hit 119 degrees Fahrenheit, airlines canceled more than 40 flights today.
Wait. What? Airplanes can't fly because it's too hot? That's crazy.
No, not really. According to news reports, the heat poses a particular problem for the Bombardier CRJ airliners, which have a maximum operating temperature of 118 degrees. Bigger planes from Airbus and Boeing can handle 126 degrees or so.

Is this solely due to the size of the aircraft, or is it a difference of manufacturer?
There are a few different things going on here. For the best answer, see this blog post.

To summarize his points, there's a combination of factors:


The Bombardiers simply aren't certified at those temperatures.
Their combination of small wings and small jet engines makes high temperatures particularly problematic for them. With the air density decreasing as the temperature rises the wings get less lift and the engines suck in less oxygen. The bigger jets have larger wings and bigger engines which make up for their larger weight so they can fly while the regional jets can't.


So to answer your question ("Is this solely due to the size of the aircraft, or is it a difference of manufacturer?"): Yes