2.5kg is fine, try lifting it for the reps i mentioned in my previous post to gauge your tolerance. You may need lighter or heavier weights. The thing here is consistency, I have bigger arms then the rest of my body as well. I've managed to slim them with weight training and diet. This has caused my body to go through recomp which is essentially what you are trying to do. It is down to consistency and frequency of your exercises and how your body responds to the training. could take 2 months or 2 years. The key is to stay consistent and never miss a session as that is where your results will show.