In Plasma Cash, users don’t need/aren’t required to download and validate the whole Plasma chain, but only the proofs related to the coins they own, as explained in the original Plasma Cash post:
Let’s say I’m an operator and I send a small amount of ETH (A) to the Plasma Cash contract and generate a coin of the same value (A) on the Plasma chain. What stops me form simply changing that coin’s value to a much higher one (B) and submit an exit? No one should challenge me, because everyone keeps track only of their own coins, and this coin is mine?
What am I missing?