Why? Aref is, by default, internally tied to Vcc, which is running off the 3.3V regulator on the Pro Mini.
You'll need to divide the 5V outputs to match a 3.3V ADC, make sure the devices use a common ground, and you're done.
Why? Because ..... well I don't know why.
I was thinking of the voltage dividers I've seen which simply divide in half. So if I did that my 0-2.5V signal would need to be compared to a 2.5V reference, right? But if I can split that 0-5V signal into 0-3.3V then that would be cool and groovy!
How do I do that? Got to be the right combination of resistors right? I could be showing my ignorance here, but the dividers I've seen have two equal resistors to ground and to the input pin with the signal in the middle. I have not done any research on this clearly. :