I'm playing Vodyani, the Leech module doesn't seem to scale in power with Ship size.
Furthermore, I tried modifying the ratio of Food leeched to Life force gained, (default is x2.5 I think, which I modded down to x1), however, that only affects the tool tips on the module.
The actual Life force income amount is still roughly x2.5 the Food leeched. 

I suspect there is a Simulation descriptor attached to the Leeching action itself that describes this ratio, rather than on the module; I found the descriptor name in EntityActions[Fleet] for "TransferResourcesInFleetAction", which would be         

<EntityModifiers Location="Self">
<SimulationAlteration DescriptorReference="FleetActionLogistics" />
<SimulationAlteration DescriptorReference="FleetActionBusy" />
<SimulationAlteration DescriptorReference="FleetActionHeroLocked" />
</EntityModifiers>

However I don't know where those 3 descriptors are to be modified...

Ideally, I want to mod the amount of Food leeched to scale with ship size Multiplier, and change the ratio of leeched Food to Life force income.

Anyone has any idea?

I don't recall the details, but is capped with the population/food of the system you are leeching from.