I think mass was mentioned somewhere in this thread, but I can't find the reference with a quick look-over of all the posts, so...
Heavier strings have more mass than light strings, so even if you adjust the break-over angle at the bridge (by raising the bridge or lowering the tailpiece or whatever) so that the bridge pressure is the same with each set of strings, the mass of the strings is still different. Heavier strings require more energy to set in motion and heavier strings can potentially supply more energy to the instrument because of their greater vibrating mass. In other words, heavier strings are not "felt" the same as lighter strings by the mandolin even if the break-over angle is adjusted for the same bridge pressure. Furthermore, considering how little difference there is in the function of a mandolin with more or less bridge pressure (within reason), it seems to me that the difference in string mass is the greater change when going from light to heavy strings or vise versa.
It is also important to consider that the pressure exerted on the bridge by the strings breaking over the bridge is a static load. The influence on the bridge made by the vibrating strings is dynamic and not too closely related to the static pressure of the strings. The two are often incorrectly assumed to be closely related.
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