At the start of the series, Stewie (at 7 months old, human equivalent of 13-14) is definitely taller than GKitten (almost 3 months old, or 3-4 years old). But that changes very quickly as they grow older.

While Stewie gets a bit taller, and a bit more muscular, he basically stops growing by the time he's one year old. GK freaking skyrockets from a foot tall to their final height of 3 feet by the time they're a year old.

To put that into perspective, GK matches Stewie's height when Stewie's a year and GK is 7 months old. That's like a 13-14 year old being the same height as an 18 year old. And they STILL KEEP GETTING TALLER.

Even though their heights vary wildly over the years, GK never lets go of their favorite past-time, hiding and/or hugging Stewie's very floofy tail.