What's the meaning of "that's saying a lot."?

The idiom lets you anchor two propositions against one another in order to add emphasis.

There was nothing I loved more than English, not even volleyball and that's saying a lot!

Think: because you know how much I actually love volleyball, so just imagine how much I loved English.

I think I'd even prefer him to the terrorists—and that's saying a lot!

This one got me scratching my head too because even prefer seems really weird when contrasted to terrorists. But the comments led to the answer. The full context is the narrator replying to someone who says: "You won't exactly be short on company". Ergo prefer him actually means prefer being in his presence.

So think: because you know how much I hate being around either of him or the terrorists, but the latter are so bad that it makes being around him enjoyable in comparison.

This has been the most fun and exciting month of my teaching career — after 14 years, that's saying a lot.

Think: because you know how much fun and exciting I've found the past 14 years, so imagine how fun and exciting that month was.

It is an idiomatic expression, say a lot about something:

• to show or express something. In general, I think the way someone dresses says a lot about their attitude. (Cambridge Dictionary of American Idioms)

In you first example, the fact that you love English more than anything else, shows clearly how much it ( English) is important or means to you. You are revealing something about yourself, your preferences.