I came to a similar conclusion from the greentext. Big and small are valuable but one is more convenient to carry around with you (due to the mass).
E: I think I see the tumblr users' confusion here... the sentence
Suppose you had $30 in coins as well, which would have greater mass?
can be interpreted like a semicolon or separate sentence...
Suppose you had $30 in coins as well. Which would have greater mass?
or as a extension describing the object...
Suppose you had $30 in coins which would have greater mass as well?
Some might have taken the latter interpretation which makes the rest of the story incomprehensible.