How important is font size consistency between pages on a website?
Very important. Consistent font sizing helps make your site look professional. When something is consistently designed, we imagine a large, established entity behind it. That gives us confidence in the site's services and content.
I have a site with about 80 pages, but some have only a few sentences; Is it important to maintain font size on these pages vs
increasing the font size to make them fill up a little more of the
You don't want to vary font size to keep content looking 'large'. It'll look strange, and users will still be able to tell there's not much content.
If you're worried about some articles being too short, maybe those pieces of content don't need their own pages in the first place? If you have lots of short pieces of data, for example, you could try and combine them into a single FAQ page, with links sending users to a particular location on the page. Each answer will be surrounded by others, so it will never look like the content is 'lost' in whitespace.