Full Idea
If there are no atoms, the smallest bodies will have infinite parts, since they can be endlessly halved. ..But then there will be no difference between the smallest thing and the whole universe, as they will equally have infinite parts.
Gist of Idea
If an object is infinitely subdivisible, it will be the same as the whole universe
Source
Lucretius (On the Nature of the Universe [c.60 BCE], I.620)
Book Reference
Lucretius: 'On the Nature of the Universe', ed/tr. Latham,Ronald [Penguin 1951], p.45
A Reaction
Another argument which remains effective even now. There must surely (intuitively) be more divisions possible in a large object than in a small one? Unless of course there were many different sizes of infinity…. See Cantor.