Good question! It is a pretty standard geometry result that if $\angle A$ is bigger than $\angle B$ in a triangle, then the side opposite $\angle A$ is longer than the side opposite $\angle B$. (This follows from the law of sines, but can also be proven using creating an isosceles triangle inside the triangle.)

The side of length 7 is in the middle in terms of length, so opposite from it must be the middle angle. Since the angles in a triangle add up to $180^\circ$, if $60^\circ$ is the smallest angle, then the angles add up to greater than $180^\circ$. Similarly, $60^\circ$ cannot be the largest angle. Hence, the $60^\circ$ angle must be the middle angle, so opposite from the middle side.