It is rather unremarkable, unimportant, or unimpressive (that something is the case) relative to someone or something else or some broader picture. True, the economy has improved under this president, but it isn't saying much if you consider how his predecessor left things. A: "So you are a better cook than your wife, who hates cooking?" B: "Yeah, I know, it's not saying much."

That is rather unremarkable, unimportant, or unimpressive relative to someone or something else or some broader picture. Well, the president has managed to get the economy to a better position than where his predecessor left it, but then that's not saying much. I'm a slightly better cook than my wife, but that isn't saying much.