Generally we measure how lethal something is by using an LD 50 that looks at mass of chemical per mass of body. There are exceptions to this, but as a first assumption it seems reasonable.
I don't think toxicity (LD 50) is relevant for plastic consumption. This sort of stuff packs into the gut and doesn't move or decompose. So the animal has to vomit it up or get surgery because it won't move down. I think the relevant comparison is ratio of pyloric sphincter between two species (the exit of the stomach). I'd be curious to know what that ratio is and it might be surprisingly close to one almost certainly less than 10. I do not know what the skill with which a whale can vomit, I do know that my lab mix pup is especially skillful.
To the same extent, the weight or mass of the plastic may not be relevant as well. The shape, density, and strength of the plastic would probably matter much more than the weight.
There could also be a selection bias, the plastic found is what was stuck, smaller parts would have gone the whole way through.