Canada Post may return a higher than expected rate for a lightweight package if it is large or bulky because they use the Volumetric Equivalent weight. Alternately, smaller but heavy items will also be impacted by this policy because in that case they will use the item's actual weight.
When shipping via Canada Post, Shippo will submit the weight and dimensions of your package to Canada Post, and the returned rate will be either the greater of the actual weight of the package, or what Canada Post calls the Volumetric Equivalent of Actual Weight (VE). You can read more about how they determine a package's Volumetric Equivalent here.
The best practice to avoid unforeseen surcharges after shipment is to submit the most accurate weight and dimensions possible when generating your labels.