For good fonts, different font sizes don't only have a different size, but the actual shape of the letters is different.
Consider the following example:
As one can see the shape of the letters is different. For the tiny font, the strokes are thicker compared to the large font. This ensures that small symbols are still readable.
If one merely scales a font, the size of the letters from the current font size is changed, but the shape of the letters is not changed.
For the best possible result, it is thus better to choose an appropriate font size instead of scaling elements that contain text.