While large animals eat more, they also eat foods that themselves have higher concentrations of mercury.

Krill eat mercury, have tiny amounts of mercury. Small fish eats many krill, bioaccumulate all the mercury from the krill. Small fish has relatively much more mercury in them than krill.

Medium fish eats many small fish. Medium fish bioaccumulates mercury from many small fish, it has even higher concentrations of mercury.

Large fish eats medium fish.

We eat the large fish, which has A LOT of mercury in it. Many times more mercury than the small animals do.

So it's not just the matter of we need more mercury and we eat more mercury but we tend to eat animals which themselves have relatively high levels of mercury. Then live long lives, giving said mercury a lot of time to build up.

I guess it's a bit redundant with the top of the food chain line.