Why would the mass of a product matter?
The US depends on importing raw resources and exporting finished products. If Canada imposes an export tariff by weight to the US, the Canadian exports that would suffer the most would be cheap but heavy raw materials while those that would benefit the most would be expensive lightweight value-added products.
For example, a 1 dollar per kg tariff on Canadian exports would be a very high tariff on unprocessed gas, but a very low one on a cell phone or computer, and it would not even apply to software.
This means that Canadian exports if bulk unprocessed products would become uncompetitive. This would push the value of the Canadian dollar down, and so help the export of lightweight or weightless high end products and services.
On the US side, it would devastate US manufacturers that would depend on Canadian raw materials to manufacture their products. Yet since it would be an export tariff and not an import tariff, the US would have no control over it.
Some US manufacturers might even move to Canada to benefit from the lower CAD since the strong US dollar would make them too uncompetitive in the US.
The US could counter an import tariff with an import tariff of its own. But how would it counter an export tariff?
If Mexico should introduce an export tariff by weight too, while US resource-extraction companies would love it, US manufacturing would be gutted. It would be like imposing the Dutch disease onto the US by pushing the US dollar up, up and beyond. All the US could do would be to extract more resources from its ground, maybe export some if it can, must mostly import high-end products that would just be too expensive to produce there.