This extract is from a popular electronics book:
... a transformer of turns ratio \$n\$ increases the impedance by \$n^2\$. There is very little primary current if the secondary is unloaded.
I'm thinking about the emboldened part. Please see if the following argument is correct: If the secondary is unloaded, its current is zero, so the secondary can be neglected (as if it doesn't exist at all), and the transformer can be modeled as a single inductor with inductance of \$L\$. And according to the relations
$$X_L=2\pi\nu L$$
and
$$V=X_L I,$$
\$L\$ must be large if primary current is 'very little'.