>If my baby does “suck down” large amounts of the same baby food every day, should we be worried or not?

This is medical advice from reddit, so... um... ...yeah:

Variety is important, from a nutritional standpoint and from a risk standpoint. Going with one brand and one ingredient and one lot number every day means that you're multiplying all the deficiencies and excesses.

Tuna can have fairly high levels of mercury, but even at 1ppm (the concentrations in the OP's report are a thousand times lower) we excrete the metals in a full serving of mercury-tainted fish in under a week. Most adult humans can metabolize/excrete more than 0.021 milligrams of mercury a day.

Maybe don't feed your child on a steady diet of only Gerber Pureed Tuna.

As for biomagnification, we're extremely complicated machines. Even with similar doses and routes of exposure, metabolism, excretion, and any negative effects are dependent on levels of things like glutathione or peroxidase, some of which are triggered by the presence and action of heavy metals themselves. It's not chemical in, chemical stays, like the diagrams in textbooks.