I've been reading about redeeming maaser sheni and I've noticed people mentioning redeeming maaser on a coin of value significantly less than the value of the maaser:
"For example, a cRc company which purchased 30,000 pounds of Israeli watermelon seeds had 2,700 pounds of ma’aser sheini seeds, which is worth tens of thousands of perutos. They were able to transfer that kedushah onto an American nickel or dime and had no need for a perutah chamurah." (http://www.crcweb.org/Sappirim/Sappirim%2025%20%28Jul%202012%29.pdf)
"A coin is designated for the redemption of Maaser Sheini. This coin must be worth at least a shoveh pruta. At the time of writing, a nickel is sufficient." [There is no mention of a maximum amount to put on the coin, nor of higher value coins being needed for more maaser.] (How can one eat non-certified kosher Israeli produce?)
And yet there is also mention of "filling up" coins used to redeem maaser: "Once the coin has some ma’aser sheini on it additional kedushah can be added up to the full value of the coin."(http://www.crcweb.org/Sappirim/Sappirim%2025%20%28Jul%202012%29.pdf)
So my question is: does the value of the coin used to redeem maaser sheni have to correlate with the value of the maaser, and if so how? And if not, why?