He says that because she talks big talk to Sabin. The conversation goes something like. . .

(SPOILER)Relm: I'm going, too!
Sabin: (laughing) Yeah! Right, kid!
Relm: What? Who is this puffed aerobics instructor, anyway?
Sabin: Kid's got quite a lip!
Relm: (mad) Acck! That's it! I'm going to paint your picture!
Terra, Locke and Strago: (freaking out) Uwaaa! No, don't!
Sabin: ???
Strago: Okay, okay, you can come, too.
Relm: That's more like it!

After that, they head back to the airship, and Edgar lags behind a little. Relm comes back to get him. . .

Relm: What's wrong, lover boy?
Edgar: How old are you?
Relm: 10. Why?
(pause)
Relm: I'm coming, too!
(runs toward airship)
Edgar: (waves) You've grown up entirely too fast. Lighten up, will you?

It might not be verbatim, but that's it.