Received a PM asking me to respond. I'll have to be brief, because we're not supposed to post or discuss OG questions!

The wording of the explanationg in OG - well, sometimes, it's not that great. Here's what's going on:

as X as = comparison language. If you see this, someone's trying to make a comparison.

Is that what's really going on in this sentence? Am I trying to compare two things? No, not really - I'm trying to give one potential characteristic of a species. That's not a comparison. So, first, I shouldn't be using comparison language at all. And that's really what the explanation should have pointed out to us.

Next, if I were trying to make a comparison using "as big as," I would have to make sure I had an apples-to-apples comparison, as others have already said above.

Finally, as someone already pointed out, we don't use "as big as" when one of the comparison halves is "30 feet long." You'd have to say something like "as long as 30 feet." That's just an idiom, unfortunately - one of those things we're supposed to memorize. But if you note in the first place that we're not even supposed to be making a comparison - or that we don't have an apples to apples comparison anyway - then you don't need to worry about the idiom issue.

Idiom.
The wolf grew up to 3 feet long.

"grew up" or "to grow up" is an idiom. It means either to get older or to get taller. It is not used to describe growing longer, which is what the sentence above is trying to convey.