There appear to be several criteria that can qualify a page as a soft 404:
- Redirect to the home page
- The phrase "not found" or equivalent in a prominent place on the page
- A blank or nearly blank page
I don't know exactly how much text you need on a page for it to be considered non-blank, but that is certainly not the usual reason that pages are soft 404. I tend to get them for redirecting to the home page. Many sites are configured to output the text "product not found" on a 200 page when a lookup in their catalog fails.
The page that you mentioned in the comment looks somewhat like an Apache error page: (image source)
My guess is that it isn't just that your page isn't large, but also that it mentions the word "Apache".
Google's John Mueller says in the comments:
If Google is getting soft-404s wrong for your site, send me some sample URLs and I'll forward them to the team here. You shouldn't need to artificially tweak the response size.
The best way to contact him appears to be through Google+.