Weight: system more important than sensor size?

At same output size and critically same detail level the D800 at 12800 does manage to be as good as the E-M5 at 3200 . Open both files in ACR { both at default ACR NR }, then add a little extra NR to the D800 { the extra detail allows you to do this } and this is the result. You can of course add more NR to the E-M5 but the problem is you do not have the same level of detail to start with and you will have a less noisy but less detailed images.

D800 12800 E-M5 3200

D800 12800 E-M5 3200