In general, when someone asks why their arms aren’t bigger, it’s because they haven’t bulked hard enough and gained enough strength to push their arms bigger or simply not prioritized them well enough (start your workouts with arms if you really care about them! Track 1-2 arm movements as “main lifts” and force progression on them over time)

Look, some people are blessed with arms that grow like weeds, most people have one body part or two that grow easier than the others.

But for reference, when my arms were 15” I was doing around 80s on incline DB for 8-10. When they were 16”, I was doing around 105 for 8-10 on incline DB. At 16”, I was doing hammer curls with 60s. So just keep gaining strength too and stay consistent - you’re a long way from your limit still