I believe the author means to use "small" as a descriptor of volume rather than of measure, and "mass" as a generic noun rather than a quantity. In fact, interp

I believe the author means to use "small" as a descriptor of volume rather than of measure, and "mass" as a generic noun rather than a quantity. In fact, interpreting "a small mass" as "something light" would be incorrect without a constant density. It's not a great sentence, because it's easy to get confused like that, but it is correct.