I urge Dev. to revert back Google sites to view in Extreme Mode, at least make this option available. The users who would like to view in High mode should switch to it, but atleast I don't want to browse Google sites in High Mode.
Allowed font sizes are indeed trimmed to some subset (which may vary between devices). In normal case you should see a difference between 12 and 19px though... Don't you have "Single column view" enabled in settings? If this is the case, then very likely page layout (including font sizes) is pretty broken.
If text hierarchy is really important to emphasize in your case, then you have few options:
try wider scale of font sizes, maybe just this 12 and 19px was an unfortunate pick,
use color scale to achieve hierarchy,
use bold text to emphasize text that would normally be slightly bigger.
If you care more about your website looking great, then you need to understand that typically websites look slightly worse in Opera Mini and our users are often used to this. If your page is readable and usable in Opera Mini, but doesn't look great, maybe that's still enough?
In any case, please note that the same page might render in different way on different devices. Especially we are working on font rendering in new clients, so actually this part might change soon (you might have a look at your page using Extreme mode in Opera Mini beta).