I know EXACTLY how you feel. Your baby is practically the same as my baby... he will be 9 months in a week and he is 12.6kg. He was going further and further away from that 97% growth path. I have met a lot of people like the other people who have replied to your post who have big babies at birth or fairly big babies but not as big as ours at their current age.
It is also my first child. I too was concerned as I had not met anyone else with the exact weight graduation my son had except for one other mother who I met in another discussion group. These include:
Birth weight = 4.3kg. Fullylbreastfed 7 times a day.
4 weeks - sleeping through the night.
6 weeks - 6.5kg. Fully breastfed 6 times a day.
9 weeks - 8kg. Fully breastfed 5 times a day.
4 months - 10.5kg. Fully breastfed 4 times a day
6 months - still fully breastfed 4 times a day and starting solids for the first time weighing 11.5kg
The early months when the average weight gain is meant to be 130g per week, my son was gaining 500-600g a week.
My clinic nurse said that he was fine because he was a happy baby who did not appear sick and he was feeding and peeing/pooing well. But at 4 months she still recommended I send my son to the paediatrician 'just to be sure' and to give us both piece of mind because it was so unusual to see a fully breastfed baby gaining weight like this.
At 4.5mths I took baby to the paed at $190! Even though I baulk at the price of the visit it was still worth it. He checked him over thoroughly and the thing he was looking for was thyroid problems. One of the main symptoms for an overactive thyroid is excessive weight/weight gain but my son did not possess the other symptoms - dry scalp and slow heart rate and as I said before he did not look sick and was a happy baby. Like the clinic nurse, the doctor could not recommend any diet alteration at this young age as he was only breastfed 4 times a day but scheduled a follow-up visit at 6 months.
At 6 months he checked him out and he was fine. The paed said I could start solids but to feed my son mainly veges and keep meat to a minimum and not to give him custards as it was a sweet weight gaining food. He also said that because I was only breastfeeding that it was my son's body's ability to hold the calories from the milk rather than me providing fatty milk or too much milk. Most babies just pee the excess calories out but my son had a much more efficient system and was storing it. A dietician was unnecessary as he was not on solids and also was a 'very healthy' infant not to be classed as equivalent to an obese solid eating toddler.
My son is now bottle-fed 3 times a day (stopped breastfeeding at 7mths) and has solid food 3 times a day (bottle still before the solid food) and we're adhering to the paed's instructions. As you can see, the weight gain did eventually slow down but he has yet to start crawling so it was not due to any physical activity although he was sitting up at 5 mths and can hold himself up to stand. He only has 5 teeth. I was told also that he will not get around easy but he is attempting to crawl now and you've also disproved that statement as your son is crawling.
My son wears size 1 clothing and Huggies XL Toddler nappies and is a happy baby. I am thankful that his weight slowed down because I can now carry him for longer periods! I am told and I am sure it is true that it is better to have a 'very healthy baby' especially for sick periods because they can 'afford' to be sick and to have off feeding days.
Piece of mind can be bought quite easily from a paediatrician if you are able. For me, I too was comforted by a lot of other mothers who said it was okay as well as my clinic nurse but I just needed complete satisfaction that he was okay.
Hope I was helpful! You can email me at email@example.com if you like.
It's all good!