It may depend on which giant breed a dog is, as they come from different molosser root stock. Boris (our EM) was 107lbs at 6 months, 132 at 8 mos, and 209 at 10 mos. The last time we weighed him (around 16 mos) he was 245. Unfortunately, we didn't keep weight charts on our GDs or Wolfies, but my impression was that the GDs grew more rapidly during puppy hood and leveled off more quickly than Boris. So the 6 mths estimation may have been more accurate for them.
Also, from reading this forum, I would say there appears to be a lot of individual variation as to growth rates.