Adjusting Harvest Multiplier in ARK

In Basic Mode

Simply adjust the value for Harvest Amount in the game settings.

In Advanced Mode

In the GameUserSettings.ini file, adjust the value HarvestAmountMultiplier=1.0.
This will apply to all resources equally.
Additionally, you can adjust the harvest amounts of individual resources in the Game.ini file.
To do this, you need to add the desired resource and multiplier to the Game.ini file as follows:

HarvestResourceItemAmountClassMultipliers = (ClassName = "PrimalItemResource_Chitin_C",Multiplier=1.0)

Command Explanation

Scales the amount of harvested resources on a resource type basis.
Higher values increase the amount per swing/attack.
HarvestResourceItemAmountClassMultipliers = (ClassName = "PrimalItemResource_Thatch_C",Multiplier = 2.0) will provide 2x the amount harvested when thatch is harvested from a tree.
Works similar to the global setting HarvestAmountMultiplier, but only for the resource type specified in this line.
Additional lines can be added for other resource types such as wood, stone, etc.