The goal is to retain the integrity of the table, which is designed to subtly emphasize part-score swings and de-emphasize slam swings relative to game swings. This was adopted when it was realized that total points scoring through the 1920's and 1940's had made part scores almost irrelevant in team play, and games much less relevant than slams.
In order to accomplish this maintenance the various types of swings must be adjusted back to the appropriate range of the conversion table. Consider the case of a vulnerable game swing in teams of four competition:
+620 and +100 => + 720 => 12 IMPS.
+620 and -170 => + 450 => 10 IMPS
Now consider a similar range of results at four tables instead of two:
+620, +620, +100, +100 => +1440 => 16 IMPS, 14 MIMPS
+620, +620, -170, -170 => +900 => 14 IMPS, 12 MIMPS
+620, +170, -170, +100 => +720 => 12 IMPS, 11 MIMPs
+620, +620, -620, -170 => +450 => 10 IMPS, 8 MIMPS
As you can see the MIMP scale successfully keeps the range of results for various flavour of vulnerable game swing centered at 11 (now MIMPS instead of IMPS) rather than having it creep up to 13.
Similar analysis of part-core and both small- and grand-slam swings would again show that the MIMP scale keeps these centered at the appropriate portion of the table.