If one takes two iPhone 6's and put them ontop of each other you've doubled the volume. These are extremely small machines already.
What is special about the chips? They're standard chips with comparative sizes/performance to their competition. They're not magical mini chips, it's not like they've compressed the power of the MBP into the MB. They're weaker.
And your point being? Would you rather carry the iPhone that is double the size and weight, or the phone as it is now? There is absolutely a noticeable difference in the size and weight of the MacBook compared to the MBP - it's a pound and half of weight, plus the larger power brick, plus the charger that I no longer have to carry for my iPhone and/or iPad. Take out the MacBook on an airplane tray table and it fits easily. The Pro? Not so much. You don't see people carrying around a Pro in their handbag all day because it's heavy enough to be a burden.
You are the one who said that the chips in the MacBook are standard size, only cheaper. That's patently false. Much of the advantage comes from their reduced size. And they aren't cheaper, they are premium parts that are designed with a different set of priorities than the chips in the MBP. In fact, for MOST use, they have done just that - the Core M is tailor-made for the bursty nature of most modern workflows, and as such, it generally performs within spitting distance of it's similar Core i5 brethren.
http://www.anandtech.com/show/9117/analyzing-intel-core-m-performance/6