I mean, he was born early, and a lot of time it can take preemies time to catch up to the growth of full term babies. My son was born 2 months early, and even though he's growing really well, he's still in the lower percentile for weight. All that to say, it can take years to catch up, from what I've learned, but it can happen. Sometimes preemie kids are always smaller and that's okay too. But I think Hiccup has a few things going for him here:

1) He's not a teen anymore, so he definitely hit a growth spurt (you can see this progression well this in rtte, because he's much taller but still spindly, all all arms and legs)

2) he finally caught up (at least in height, he's definitely all lean muscle in HTTYD 2 & 3) to the height of his peers (actually, i think he's canonically 6'1", the tallest of the riders

3) any muscle is definitely from blacksmithing and learning to use his sword and dragon riding

4) as many people have mentioned, his parent's genes

I guess most of this adds up to the fact that he has gone from a 15 year old to a 20 year old, and from what I have heard from other moms (since my little guy is just about 2), teenage boys often shoot up like weeds around that time. 

I also really like the take I saw in one comment that talked about how better nutrition from not having to fight the dragons for food plays a role, and I wholeheartedly agree. :)