I can not remember what DS weighed at that aged, but he too stacked it on but have always been assured he's not over weight....so dont stress. At 7months Charlie was 9kg & now at almost 10months he's 10.3kg so he has not put on a heap in the past few months. I would say due to crawling, but he is also a whopping 78cm long!!!
I get the "oh how old?" & "oh Charlie, you just keep growing"....."gee he's a big boy isn't he". It used to get to me but all babies are different so now I just let it go.