Full Idea
The concrete, particular object actually is a composite not just of matter and form, but also a large number of accidents, like size, weight, colour. So we should not assume that the composite of matter and form is identified with the concrete particular.
Gist of Idea
Form and matter may not make up a concrete particular, because there are also accidents like weight
Source
report of Aristotle (Metaphysics [c.324 BCE], partic) by Michael Frede - Substance in Aristotle's 'Metaphysics' p.74
Book Reference
Frede,Michael: 'Essays in Ancient Philosophy' [University of Minnesota 1987], p.74
A Reaction
That gives a nice well-rounded picture of how we should understand a physical object, to fit it into the rest of our conceptual scheme, and the way we think about it.