My .riv file is of 421 KB !

Hey! I responded to the Jira ticket you submitted, but I'll respond here as well.

The reason for this file being so large is that you are including the entire font when exporting. If you don't plan to localize the file, then you don't need to include all of the glyphs. To remove those glyphs, select the font from the assets panel, then in the inspector scroll down until you find export options. From there, you select the option to include only glyphs used.