I’m a bit surprised there doesn’t seem to be some nerd that has made like a tvtropes list with all damage types so I only though about the types I found
resists poison because I’m thinking it is a % in blood/body thing and if there is more blood/body then it is harder to get to whatever % is dangerous
resists bludgeoning because fat is built in padding
resists air/wind because you are heavy and round/aerodynamic
resists healing because there is more person/body to heal compared to a person with the same max hp but less fat
weak against bleed (and I guess slashing for a similar reason) because you need more or something (more of a “post hock” rationalistic for just a feel that “fats takes more bleed damage because it should”)
weak against pokemon ground because you are more “grounded” than someone lighter
weak against pokemon fighting because having your weight used against you is a thing (I think)
fatigue status effects would be easier to receive
If accuracy is a thing, fat could reduce (or eliminate?) the miss chance. That’d make Hail-Mary attacks a reliable counter to Fat Types - if the Rainbow Slash has a 10% chance of inflicting each elemental status type, even having a resistance wouldn’t help much when you take on every status debuff in the game all at once.
id say plant, or fungus if its its own element cause i think it should be, would be strong against fat, cause damp folds and sweat are the best environment for funghi to grow in
Pros
Resist cold because of insulation.
Resist earth because most earth moves would do blunt force damage and Fat is padding.
Resist Stun effects from blows because padding.
Partially resist poisons because of Body mass.
Cons
Weakness to wind because they have larger surface area and would be blown more easily.
Weakness to fire because heat retention and fat might melt and continiously cause burns.
Accuracy against the fat target would be increased because larger target.
Speed boosts would have only 75 effectiveness on a fat target.
Stealth moves are harder for Fat characters to do.
Misc:
Guard moves against other party members work more often as fat character put themselves in the way of their team mates.