We were initially skeptical about reports of a 13-inch Retina MacBook Pro, but we are obviously believers now that we have received word from very reliable sources that they are due in just over a week. Why were we skeptical?

Apple already has an incredible form factor in the 13-inch MacBook Air. So, why not simply give it a Retina Display?

It turns out there are a few reasons, but let’s start with the Retina display: the Retina Display on the 13-inch MacBook Pro will be 2,560-by-1,600 (for those scoring at home, that resolution is the same as the discontinued 30-inch Cinema Display) and is certainly a lot of pixels for a mobile video card to drive. Sure, the Intel 4000 series chips currently powering MacBook Airs can drive that many pixels; but if you have ever hooked your MacBook Air up to a 27-inch or 30-inch display, you know it is not a good way to live your life —especially not for ‘Pros.’

So…to drive a 2,560-by-1,600 Retina Display, you need a discrete video GPU which requires some extra room on the motherboard and some extra battery power to maintain mobile usage benchmarks. Much more of each than an Air can currently hold.

We also know from the heavier/thicker third-generation iPad that driving the LEDs behind a Retina display takes a lot more power and therefore battery than a regular display.

Those batteries wouldn’t just need to power a discrete video processor and a display. They also can power a much beefier CPU. The highest-power CPU you can put in an Air is currently the BTO 2.0GHz dual-core Intel Core i7. It is really no contest when compared to the 2.7GHz quad-core Intel Core i7 high-end option for the MacBook Pros. That processor also requires a bigger heat-sink and fans with outlets on the bottom of the machine.

MacBook Pros also can offer twice the RAM (16GB) as Airs, which is also a differentiator and does take up some motherboard space.

So, when it comes down to it, the reason to build a Pro is almost completely about space requirements. We had heard whispers at the time of the 15-inch launch that there was a 13-incher ready but there were heat and battery life issues. We imagine it took Apple a few months to iron those things out. What is certain is that there isn’t enough space inside an Air enclosure to create a ‘Pro level’ machine.

There is something important to keep in mind if you are thinking about making the switch for (or choosing between) a 13-inch Air to 13-inch Retina MBP: You will lose significant usable desktop space when running in native Retina. Remember, the 13-inch Retina display is giving you the usable space of a 1,280-by-800 display. The MacBook Air gives you a regular 1,400-by-900 display—the same usable space as the 15-inch Retina MacBook Pro.  Of course you can dial your Retina display all the way up to a native pixel resolution, but most people will stick at Apple’s Retina setting.