How to discuss coefficients in big-O notation
Big-$O$ and big-$\Theta$ notations hide coefficients of the leading term, so if you have two functions that are both $\Theta(n^2)$ you cannot compare their absolute values without looking at the functions themselves.  It's not wrong per se to say that $7x^2 + 4x + 2 = \Theta(7x^2)$, but it's not informative because $7x^2 + 4x + 2 = \Theta(3x^2)$ is also true (and, in fact, it's $\Theta(kx^2)$ for any positive constant $k$).

There are other notations you might want to use instead.  For example, $\sim$ notation is a much stronger claim than big-$\Theta$:

$\qquad \displaystyle f(x) \sim g(x) \iff \lim_{x \to \infty} \frac{f(x)}{g(x)} = 1$

For example, $7x^2 + 4x + 2 \sim 7x^2$, but the claim $7x^2 + 4x + 2 \sim 3x^2$ would be false.  You can think of tilde notation as $\Theta$ notation that preserves the leading coefficients, which seems to be what you're looking for if you do care about the leading coefficient of the dominant growth term.