In my 7D and 700D, I always used to set my distance on the 100mm macro lens to 1:3 (1.52ft) to take intra-oral dental images. For potrait images I set the lens to 5ft. I take these images on a Manual setting
I have changed over to the 6Dii, the same settings makes the images appear at a further distance. I have to set the lens to a 1:2 distance to get the same effect for intra-oral effects or stand closer to the subject to get the desired potrait shots
Please is there some setting that I have not correctly done? Please can someone kindly help as this is driving me nuts?
Your explanation is correct. The 6DII is a full frame camera whereas your previous cameras where APSC crop cameras. What this means is that sensors on you previous cameras were smaller which has the effect of only showing part of the central area of the image that you would obtain with a full frame camera, this gives the impression that the image appears more magnified with a crop camera.
There is a lot of information about this on the web and if you carry out a search for digital sensor size you should find plenty of sites that will explain it better than I have.
The difference is actually in the lens not the camera. You will need to use a 160mm macro lens to get the same perspective as you had before with the 7D, 70D. However, not wanting to go that route you can probably enlarge or crop the 6D2 photo to the same perspective and possibly get even better photos.