Larger animals can eat more of a poison without getting affected. Mice can eat somewhat more theobromine than humans per weight, but humans can eat far more chocolate in raw weight. So to say that how large an animal has nothing to do with it is completely misrepresenting the truth.
You mistake not describing absolutely everything for saying nothing else matters.
Larger animals can eat more of a poison before being affected. That is truth. It's just a simplified truth, considering we have no data of some hypothetical larger animal that can eat 400 kg in a day and whatever theobromine metabolism they have. For this manga, that's probably dragons, and they tend to be very hard to kill in most ways, including poison.
In practice, there are more complex things happening that affects the final result, so assuming a simple solution is just not thinking
Now you're really straying into territory that you have no idea about.
Saying a larger animal can whilst not even calmly accepting the fact you spoke a half truth and then trying to justify it is hypocrisy,because in order for something to be easily stated by itself,the exact opposite must also be true,which is not since rats can consume a fairly large amount of theobromine compared to their weight.
And you've even went out of your way to further generalise it to "of a poison",which can mean any poison,which immediately makes it wrong since the lethal doses for some of the most potent poisons are so small that the difference between killing a rat and killing an elephant is now a time measurement,for it's harder to even make the lethal dose for an a elephant small enough to be the minimum to take out a rat but leave the elephant alone,one could with mid and lower tier,but "of a poison" still includes higher tiers.
Simply put,the weight of an animal is not the deciding factor(if a factor at all)it's a false positive that only reinforces biased results,therefore it should not be used as the factual statement,for it causes flawed logic that only worsens things.
If you do not describe it to the best of ones' ability,you're no better then someone who claims nothing else matters.
Also,saying a human can eat more chocolate then a rat falls apart when a dog can eat more then a rat,the former doesn't die from the same dose that the latter does,despite the existence of dogs that can weigh and measure as much as a healthy human,and that's without mentioning human children.