If atoms can create molecules, planets can create solar systems, stars can and do create galaxies, and those create what's in the universe, then where does it end? What's physically biggest, and how does that theory 'hold water'?
By your request to know "how the theory 'holds water,'" I must assume you intend to treat the answer with great scrutiny. Thus the only answers that can be given are those which are rigorously true.
The biggest thing that can physically exist in this universe is the universe
If you do not accept that an entity can exist within itself:
The biggest thing that can physically exist in this universe is the universe minus the smallest thing in the universe
A more philosophical answer would be to look at the hierarchy you chose, and break it down. You describe a physical ontology based on collections of mass-ful objects. Atoms make up molecules [which make up planets] which make up solar systems which make up galaxies which make up the universe. However, this premise is inherently flawed because it is missing several key details
- There is more to the universe than its mass-ful particles. Photons are in the universe and do not fit into this ontology.
- The ontology does not account for all characteristics of its particles. Consider the organization of molecules: 2 hydrogen molecules (H2) and 1 oxygen molecule (O2) are not the same thing as 2 molecules of water (H2O). The former, when lit, set off tremendous energy before becoming water, so there's more to them than just their mass.
- There are mass-ful particles which are not included. Comets are not included in your ontology.
Finally, and most importantly, each of those categories is a human defined category, not a naturally defined category. For example, it is convenient of us to think of materials as molecules, but within the solar furnaces of our sun, it is too hot. On the boundaries of being molecules, we have atoms attracted by hydrogen bonding, and so forth. Thus this sequence of categories ends whenever it is no longer convenient for humans to give a name to a larger structure.
Even "the universe" itself, from your question, is a human defined topic. Whether it contains "everything" or if you limit it to the portion of everything which we know how to measure, it is simply a boundary. It may be the final boundary, or it might not. Only time will tell.