
I'd expect that sort of weight at 3 months. Surely the OP means 16kg????
My breed is stockier than a WS but I would say a similar size, and their height seems to be not much less than most I see these days (even though standard says different). anyway I would expect my medium size breed to gain all but half an inch of adult height by 9 months, and would not think A Border Collie would be much different, though they will both fill out and mature over about 3 years or even longer.
As regards food needs changing this usually means it stops increasing and often people reduce the protein and fat content to avoid overweight.
With my own breed I often find this is the age at which many pups can tend to get fat as the owners haven't notice their pups food needs levelling out/decreasing. My breed though are generally good doers and often very food motivated, and I don't think too many collies would be getting fat until much later.