please criticise-deduced fact about entropy of substances with varying mass/atoms

No, not really.

Think of entropy in terms of "disorder", try to gauge the disorder of the products vs the reactants, then being able to predict the entropic changes in a reaction.

In terms of order; solid<liquid<<gas.

Be comparing the moles of species in each state.

But what if we are comparing two substances in the same state, both with same no. Of moles but different mass and no. Of atoms in them.?

They will have different entropies, but not BECAUSE one has more mass.

And not necessarily because things have more atoms, although there can be a correlation.

Entropy is a measure of order and probability. Larger molecules often are more "disordered" because they can adopt more shapes in space. This is not always the case though, it depends on the molecule in question. Hence why "the bigger the molecule, the greater the entropy" is not a good rule. Simply a reasonably common consequence to a rule that, at A-level, you probably don't need to know.