Certainly the amplitude of a harmonic can exceed the amplitude of the fundamental,
or the amplitude of an intermodulation product can exceed the amplitudes of the individual signals. But this occurs when the system is so nonlinear that it would
make no sense to use it as an approximation to a linear system.
Indeed, the very
fact that such a nonlinear system is being used (intentionally)
means that it has some properties
that are useful in the application at hand.
Example: If the system is a squarer that produces output $x^2(t)$, then
with $x(t) = \cos(2\pi f_c t)$, the output is entirely the
second harmonic (and a DC term). Since the fundamental is entirely
absent from the output, the harmonic amplitude is infinitely larger
than the fundamental. If two sinusoidal inputs of frequencies $f_1$ and
$f_2$ are added together before the
squaring operation, then the output has second harmonics at frequencies
$2f_1$ and $2f_2$, intermodulation distortion products at frequencies
$f_1+f_2$ and $|f_1-f_2|$ and no fundamentals at frequencies $f_1$ and
Moral: Don't use a squarer and pretend it is a linear addition circuit
or linear amplfier. If you are knowingly using a squarer, it is for
some other useful properties (e.g. frequency doubler) of the circuit that
are important to you right now.