Wear often actually does not reduce weight that much percentage wise.
Poltorak / Groschen coins just where struck with quite huge variant i. Weight. Standard was not as strick as say with Taler coins or Dukat which followed stricter measurement limits.
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I think that the weight requirements were met for gold coins, or large denominations of silver. Bilon small coins, where the silver content has decreased every year (and pure silver has the highest weight), has a large difference in weight. In addition, the coins were physically worn out and lost weight.
Here are some SIGISMUND Poltirak examples with the lowest weight.
So, yes the silver content was changing, but the weight was also intentionally decreasing according to the mint specifications.
As correctly pointed out by apuking, wear cannot reduce the weight of a coin by 40%. The coins in walter_scott's photo look to have been either struck on small flans, or were clipped, especially the first one which is not round (and remember that reducing the diameter by just over 10% reduces a coin's weight by 20%).
In 1623, the mint rate was changed and 168 półtoraks were minted from the Krakow fine of silver. Previously, it was 128. The ordinance stipulated that 168 półtoraks were to be minted from the fine. It did not determine the weight of a single coin, but stated how many coins should be minted from a fixed amount of ore, or specifically how much 168 półtorak should weigh. We call this system for determining the weight of coins "al marco". The name comes from the Latin name of the fine (March). For heavy coins - ducats, thalers - a different method was used, which consisted in the precise weight control of each disc. If necessary, too heavy discs were filed, and too light ones were sent for remelting. The plate for półtorak was of different thickness. Therefore, the weight of półtoraks varies, and the thicker or thinner disc did not matter much for the producer. Another is the corruption of money, when the edge of the coin was cut for profit in order to obtain silver. My statement related to the coin in the photo. If I attach a link to a coin in MS condition from 1623, the weight of which is 0.94g, then we are not talking about a consumption of as much as 40%. Here the percentage of consumption is much lower. And the coin in the photo is not cropped.
