m4/3 cameras and lenses are definitely smaller and lighter than a DSLR, despite what others have said. This is especially true if you get the cameras that don't have a grip or viewfinder.
The difference is more noticeable the larger the lenses get, too (for an equivalent field of view)
Here is a link with 3 micro 4/3 cameras and two Canon DSLRs. The pictures are form above with lenses attached.
http://camerasize.com/compact/#570.397,472.335,482.34,333.377,449.26,449.310,525.35,ha,t
From left to right we have a Panasonic GM5 with a tiny 12-32mm lens on it. That's equivalent to a .85-2.67x zoom lens. It's very very small.
Next is a Panasonic GX7. Very hight quality and moderate size. It has a collapsible 14-42mm lens on it (1-3x)
Third is an Olympus E-M1. It' a fantastic camera. It's shown fitted with a Panasonic 14-140mm lens. You can swap Panasonic and Olympus lenses freely. It's the standard versatile zoom from Panasonic (1-10x)
Fourth is a Canon T4i. It's their basic DSLR. It's shown mounted with the kit 18-55mm lens. (1-3x zoom). As you can see, the package is much, much larger than the GX7 that has the identical field of view range and it's even larger than the Olympus mounted with a lens with much longer reach!
Lastly is a Canon T5i mounted with the standard versatile zoom of 18-200mm.
Total volume of the cameras, in cubic cm, is roughly:
GM5: 212
GX7: 473
EM1: 769
T4i: 1047
T5i: 1047
Weight, in grams:
GM5: 211
GX7: 402
EM1: 497
T4i: 575
T5i: 575
Approximate depth with shown lens attached, in mm:
GM5: 60.1
GX7: 81.4
EM1: 147.1
T4i: 154
T5i: 241.3
Weight with lens attached, in grams:
GM5: 281
GX7: 497
EM1: 957 (the lens is quite heavy. one of the heaviest in the system. The second version is much lighter)
T4i: 780
T5i: 1170
Mirrorless gives you more options for camera styles as you don't need the mirror box of a DSLR, with some cameras being very small. That GM5 is 1/5 the weight of the DSLRs shown. The sensor is smaller, so the lenses can be smaller. But, thankfully, the sensor isn't so much smaller it seriously affects the imaging quality. In addition, the flange distance is less so lenses can be shorter.
Toss a superzoom (100-300mm) with the standard kit zoom (14-42mm) with micro 4/3 and you're looking at a $600 lens that is about 530 cubic cm and 520 grams.
Toss an equivalent Canon superzoom, which would be 130-385mm (and doesn't exist so I chose the 100-400mm instead) and you're looking at a $1700 lens that's 1233 cubic cm and weighs 1380g.
The comment in the first response that "A mirrorless 4/3 isn't going to be all that much less bulky for carrying around on long trips if the idea is to be carrying a bunch of camera gear, multiple lenses, etc" isn't borne out when you actually look at the cameras.
Oh, heck. Just for fun I added two more cameras to the link. On the far right (camera 6) is a T5i with the 100-400 attached and a Panasonic GH4 (which is the largest m4/3 camera, I believe) - camera 7 and the very last one shown - with the 100-300 attached (the largest m4/3 by volume, I believe).
Here is the link again:
http://camerasize.com/compact/#570.397,472.335,482.34,333.377,449.26,449.310,525.35,ha,t