Another consideration which may have implications on font size and licensing:
Variable fonts may contain “named instances”, which correspond to the static font styles we have today: ‘Bold’, ‘Light Condensed Italic’, ‘Serif Caption’ etc.
But with access to font variations in CSS, there will be no need to include named instances in webfonts: they can be defined directly in the CSS. The foundry can deliver a small variable font (masters only) and a CSS snippet with the instance definitions (mapping of names to coordinates). Web designers can then fine-tune instances directly in CSS, if needed.