Call Quality While Using a Bumper w/ IP5

Has anyone found that people have a harder time hearing what you're saying during phone calls when you use a bumper case with the iPhone 5? I've consistently experienced people asking me to speak up or telling me that it's hard to understand me.

Here's my hypothesis as to why that's happening... Because the IP5 is taller, your mouth is further away from the mic at the bottom of the phone. When you put on a bumper case, it partially blocks your voice a bit, causing the issue with people not being able to hear you as well.

Any thoughts, anyone? Am I the only one experiencing this?