There may be some correlation between iPad size and disposable income. The bigger the iPad, the wealthier the owner. That translates into more money to spend on flying and more hours in the air, so more confidence.
Alternatively if all the pilots had similar financial resources, the size of the iPad could relate to their commitment to flying as it is a significant investment.
As for the reason to pick an iPad over an Android device, Android devices are more prone to computer viruses and software incompatibility between operating system versions. Both types of tablet are capable of doing exactly the same job, but for safety critical applications, it has to be an iPad.