Yes but they have larger bodies and lenses, which brought me to the CX. Otherwise, I may as we'll carry my D600
I think you'll be surprised. Screenshots from camerashots.com
• Nikon D600 is 8% (10.6 mm)
wider and 21% (19.5 mm)
taller than Olympus OM-D E-M1.
• Nikon D600 is 30% (18.9 mm)
thicker than Olympus OM-D E-M1.
• Nikon D600 [850 g] weights 71% (353 grams)
more than Olympus OM-D E-M1 [497 g] (*inc. batteries and memory card).
• Nikon D600 dimensions: 141x113x82 mm (camera body only, excluding protrusion)
• Olympus OM-D E-M1 dimensions: 130.4x93.5x63.1 mm (camera body only, excluding protrusion)
The OM-D EM-1 has a weather sealed magnesium case like the D300, D4, D800, etc.
Here are a couple comparisons to your D600 and J3: