I feel like there is no perfect word to describe the opposite of “dense.” Density as in mass divided by volume.
All I can think of are words like “light,” “fluffy,” or “sparse” that aren’t exactly what I am trying to say when describing something that has a large volume, but not a lot of mass.
Of all the bowling balls he preferred the [not dense] ones because throwing them down the alley fast made him feel cool.
A historical word to mean the opposite of dense was rare, as stated in the following taken from an article in DON’T LOOK NOW
IT’S DEVAN GOLDSTEIN:
Once upon a time, though, dense had an opposite. Until the mid-19th century, the word was rare.
In fact, the earliest meaning of rare in English was precisely this one. The OED notes the word’s meaning as “[o]pposed to dense,” and provides the following example (from around 1420), among others: The londis fatte, or lene, or thicke, or rare.
Also, a check of current dictionary definitions gives us this one from Merriam-Webster:
rare: marked by wide separation of component particles; thin (e.g. rare air)
So, rare does seem to be used for “objects of low(er) density” in the physics sense of the word. This is because typically when we think of “dense” we think of “heavy”, but in the pure physical world this in not the case as weight is subjective, while (as you mentioned) mass and volume are not.
Now the caveat to all this, is that the usage of word this way in now rare itself (pun intended). It quite possible that you will get a odd look or be misunderstood if you ask for a “rare bowling ball”.