They mean "mass". - for example, a cubic meter of feathers has less mass than a cubic meter of lead. They occupy the same volume and maybe have the same dimensions, but their masses are different.

CJSF

__________________
Two years ago moved from my town
I was looking up past the city lights
But the city lights got in my way

See the constellation ride across the sky
No cigar, no lady on his arm
Just a guy made of dots and lines

-from "See The Constellation"
by They Might Be Giants