Use MapSizeDefs.xml for your mod

Go near the end of the file and find the insane map size.  Here are the original values pre-1.9 (changed lines are bolded)

<MapSize>
<InternalName>Insane</InternalName>
<HexSectorRadius>0</HexSectorRadius>
<HexSectorSize>380</HexSectorSize>
<Base>1200</Base>
<MaxHabitablePlanets>600</MaxHabitablePlanets>
<RiftAppearanceChance>0.00</RiftAppearanceChance>
<MinPrecursorPlanets>16</MinPrecursorPlanets>
<PrecursorPlanetPercentOfHabitable>0.01</PrecursorPlanetPercentOfHabitable>
<StarboxDef>DefaultStarbox</StarboxDef>
<MinCameraZoom>100.0</MinCameraZoom>
<MaxCameraZoom>18200.0</MaxCameraZoom>

Let me know how this works for you

This mod restores both the galaxy size definitions from v1.83, as well as the default object frequencies (planets, stars, etc.). If you want to use the new object frequencies with the old (larger) map sizes, just remove the file 'MapSetupDefs.xml' from the mod folder.