The *preview* correctly scales down from the [default] size, in this case "-2".
The saved version does not. They should be consistent, the holy mantra of 'logic'.
(Can be handy in sub- super- scripts re formulas, for just one legit matter)
If the display fonts cannot display smaller than the default font, why should the preview imply They Can? (and obv. Do - in preview)
Pshaw