Activating voice control rapidly heats up device

I upgraded from the iPhone5 to 6s to get access to the improvements in voice recognition. I often use my iPhone for dictation drafting as it comes in handy when fingers can't keep up with the mind xD

Voice control offers non-interrupted voice recognition making it my preferred approach to dictation.

However, whenever I activate voice control my device rapidly heats up to a point where holding it in my hand gets - not painful, but very unpleasant. At that stage I feel forced to disable the feature as to not risk damaging components by overheating. It seems contradictory though and I would like to ask whether or not this is a common issue, or if it's something that requires attention?

OBS: This happens as soon as it's activated whether you actively use it or not. I've had the same result after activating voice control, then put it in a fridge for 10 mins and still come out hot...?