@MR&R,
You don't have nearly enough data make a statement that the the group size is going from large to small with increasing distance. You do have enough information to make the statement that you see this happening based on anecdotal evidence. If you study enough about external ballistics you will find that the proposition you propose isn't valid. This is not to say your groups didn't do what you say they did.
As to data if you shot all these groups as four shot groups then you threw out half of your data by looking at only the two extreme shots in each group. When you are shooting at 400yds and 1000yds you are dealing with numerous external factors that affect the flight of the bullet that create noise (errors) in the intended point of impact. In a case like this trying to prove the precision of the round on target it makes sense to utilize all shots and to determine the mean radius of the group. In comparing one group to another this method allows you to determine a standard deviation of each group and this allows for use of statistical tool to determine if the differs are statistically significant. To be useful you would need to shoot 20 shots eat each distance to have reasonable confidence in the results.
It is much more likely that the results are, in fact shooter induced. If you are shooting a 1000 yd target with a 1.2" X ring at 400yds that is equivalent to a 3" X ring. Vastly different sight picture. This difference in sight picture MAY lead to the mind viewing the target differently and causing changes in how the body responds to attempts to duplicate what is needed to break the shot. Ideally you would like to have ~.5" X ring at 400 yds. In this case taking a 40% Reduction of the target center would help eliminate sight picture effects.
You might find Hornady Podcast #50 of interest.