Nah, X's height should remain 160 cm, even in the ZERO-series. Same for Zero, but I guess he just has a very youthful appearance:
TERRY: Hey there, kiddo. It's awfully dangerous around here for a tyke.
ZERO: Ugh... Watch it...! Kiddo?
TERRY: You exude confidence. I like that. Let's mix it up!
KEN: Hey, you're about as big as my boy back home. That gives me pause.
ZERO: ...Enough. ...I am no child.
KEN: ...OK. No guilt here. Let's do it, sonny! I'll teach you that it's a good
idea to be polite to elders!
CHUN: Excuse me. I'm looking for this guy, can you help me?
ZERO: No... I'm in a hurry...
CHUN: Have you seen a big guy around here with red hat and clothes?
ZERO: Can't help you...
CHUN: You're a pretty snotty little kid. ...Hmm. What's that? Are you packing?
Show me that weapon.
ZERO: Don't touch me...!
CHUN: What's with you? I won't pamper you just because you're a kid!