The context I was referring to is the assumed purpose of the Great Philosopher's use of the metaphor & the assumed scope of the comparison between bills and coins to be of their representative money denomination only. You have acknowledged in your explanation that both are equally valuable despite their differences in their mass, but this same qualification is not included in the OP and that's the source of the confusion. After the difference is mentioned in their penultimate question, the word 'but' is used as a soft indication of an ultimate answer converse to the previous answer, coins, which have greater mass. In the text and subtext, the use of this word is the first and only indication whatsoever of the Great Philosopher's implication and answer. But with this info alone it still is still equally possible that the Great Philosopher's point is that both $30 of bills and coins are of equal value and therefore, both big and small boobs are of equal value; or that bills subjectively have greater value as a result of their lower mass and therefore that small boobs are greater in value than big boobs.

This is not further clarified in the text. You can use your relevant formative experiences to figure out the intended point, probably more than 50% of the time. But if you posted this on a small boob enthusiast forum, everyone there would understand this meme to be justifying their enthusiasm about small boobs.