The size tag, just like every other text formatting tool on these forums, is active because it can be used legitimately in actual text formatting. For instance, a lengthy guide could use big text for section headers in order to separate different topics of content.
None of the text formatting tools on these forums are meant to be used as attention-grabbers, because if your post is interesting and contributive enough people will read it without being forced to notice it. Just because a tool is there does not mean you're allowed to do anything with it.