..... there is a case to be made that the outer dimensions be as per ICFRA - 2400x1800mm and 1800x1800mm. There are a number of other reasons to justify consideration of this. One is that it would reduce the target weight a little bit.
One Target Rifle reason would be for those that Frame the target when sighting. I used to do this past 600 yards. All of a sudden your ring sizes are no longer correct and would need to be bigger
Likewise, if you shoot on one of those 1800 x 1800 targets.
13.1.2 There are three Classes of target:- (a) Third Class 1200mm x 1200mm square. (b) Second Class 1800mm x 1800mm square. (c) First Class 2400mm x 1800mm. (d) When electronic targets are in use it is permissible to use targets with First Class or Second Class dimensions at all distances.