Thank you joeball123! That's what I thought, and just so everyone knows, the value is Growth in GalCiv3GlobalDefs.xml is as follows:
<GlobalColonyMods>
<EffectType>Growth</EffectType>
<Target>
<TargetType>Colony</TargetType>
</Target>
<BonusType>Flat</BonusType>
<Value>0.05</Value>
</GlobalColonyMods>
The default value is 0.2 and I turned it down to 0.05.
Turns out that the main source of my confusion is that this is the value I was editing in my mod but the effect wasn't taking within the game. I have mods enabled and the other changes are taking effect. I had to make the change within the game directory in steamapps. This doesn't seem right as it will be overridden during updates. Is GalCiv3GlobalDef.xml not editable within a mod?
Within the mod folder the path to the xml file is as follows:
~/data/Game/GalCiv3GlobalDefs.xml
Thanks,
Joe