[Wendigo]

As arch says, supply is based on size: your lobsters eat as much as 5 size2 troopers, and your shamblers eat twice as much.

Solve your upkeep problems with a better economy: better economic scales (order, production, growth & luck), a higher admin fort, building forts in your best provinces income-wise, searching for fire-earth gems & alchemizing them with an alchemist stone)

Solve your supply issues with a growth scale (increases supply inside your dominion +20% per pick, and with further increases over time because of population increases), and by picking a good admin fort & building it in those provinces you have supply problems (forts have a supply radius).

A lot of people play nowadays with low admin forts & a death scale, maybe that just doesn't suit you (it's definitely not a 'must do' as far as I am concerned).