Big thanks to Nigel for writing this in answer to a question posted here

Note: posted August 2014.
Quote Originally Posted by nigel View Post
First you have to decide what sized sensor you want and need.

Full frame (biggest), only Sony here.
APS-C, Fuji, and Samsung.
Micro four thirds, Panasonic and Olympus.
Nikon 1 series.
Or the Q series by Pentax (smallest).

Your biggest weight is the lenses, so a really small body (eg, the Sony A7) is not much weight saving against another full frame camera.
The smaller the format the small the lenses can be made.

Any mirrorless with the same sized sensor as a DSLR, will equal it. If the mirrorless has a bigger sensor, then it should beat it.
But if you put high end glass on a smaller format, then it tends to be dam close.

Do you want DSLR styling, eg; Olympus EM1, or Fuji XT1.
As most mirrorless are styled as older rangefinder (or point and shoot) cameras.

Do consider what lenses you might need, some systems have very little in the way of a lens range (Sony), while others have a huge selection (micro four thirds).

If you want small I'd be tempted to have a look at an Olympus PEN series camera the EPM2, with the twin lens kit.
The basic walk around lens is a 14-42 (2x crop, 28-84mm, 35mm equivlent.), and the camera is almost no bigger than a point and shoot.
You get a huge sensor against a point and shoot, and more options for manual control.
The only down side, is the manual controls tend to be hidden or hard to get to, as this camera is designed for the full auto crowd. And no viewfinder, just the screen on the back.

With any lens selection, a pro lens will be bigger, heavier, and more expensive. If your trying to keep it small, I'd stick to consumer lenses.
But get your hands on any of the above brands as they all vary in size and layout. Some will feel better than another.

Much to consider, good luck.