This is about interpreting metaphors and not reading comprehension.

Yes, there's some reading comprehension issues here, but there's also bad writing.

The original question is about size, but the Philosopher, for some reason, makes a detour into mass. This detour goes nowhere, and just ends up as a distraction to the point he's trying to make. He could have just said, "Suppose you were to have $30 in coins instead, which would have more value, the coins or the bills?" No introduction of "mass" for no reason, just a straightforward analogy that different things can have the same value. Or, he could have kept the idea of size: "Suppose you needed to carry $30 in coins instead, would you need a bigger wallet? ... Ah, but which wallet's contents would have the greater value?"

It's also distracting that he says "you were to have $30 in coins as well". That makes it seem like it's important that Anon now has $60 instead of $30. If the idea was to compare $30 in coins to $30 in bills, a better wording would be "instead". Then you're comparing two situations in which Anon has $30, instead of a situation where he now has $60 instead of his original $30 but half of it is now in coins.

The way it's written is like a trick question where the obvious answer is wrong. The obvious answer is right, it just feels like it's wrong because it's badly written.