So if it's "opinion-size-age-shape-colour-origin-material-purpose noun", why would you say "the big bad wolf", but also "the bad old days"? In the latter, 'bad' is acting as opinion. But what's it doing in the former? Purpose?
To borrow some parentheses, (the big (bad wolf)) highlights the character of the wolf as being more important than it’s shape. However (the smelly (bad meat)) emphasises the bad-ness, or rotten-ness, of the meat and further describes it as smelly.
Similarly the focus of (the bad (old days)) the past, and bad is acting as an adjective for (old days).