CoreUnitStats.xml
<UnitStatType InternalName="UnitStat_TacticalScaleMultiplier">
<DisplayName>Tactical Scale Multiplier</DisplayName>
<Description>Scale multiplier for this unit when in tactical battles.</Description>
<Hidden>1</Hidden>
<DefaultValue>1.0</DefaultValue>
<UnitStatGrouping>CombatStat</UnitStatGrouping>
</UnitStatType>
Change defaultvalue to something lower. Alter the shrink/grow spells as you see fit.