On a pretty regular basis this season (and all seasons) I take measurements of my pumpkins and then use growing charts to get an estimate of how much my pumpkins weigh. These charts are typically fairly accurate but sometimes pumpkins will go as much as 30% over the charts and sometimes as much as 20% under the charts. For the most part, most pumpkins will be within 3% of the estimated weights on the charts.
Measuring the pumpkins regularly helps me know what environmental factors are affecting the pumpkins growth and sometimes helps me identify problems early, but mostly I just want to know how big my pumpkins have grown (it is a bit of an obsession). With all of that however you have to keep in mind, come weigh-off day, that the pumpkin's weight on the scale could be much different than what you had anticipated.
I remember looking at the faces of the top three pumpkins at Topsfield in 2008. Would you have guessed that the pumpkin on the left took 3rd place? It ended up 150lbs less than the pumpkin on the right. If the picture was larger you would see that the growers on the right were smiling and the grower on the left wasn't. His pumpkin went 10% lighter than what the charts said it would. I've been there. A few years ago I had a pumpkin go 15% light and I didn't see it coming. It can leave a bad taste in your mouth for months so be prepared for the scale says, because it doesn't lie.
The piece that you can never tell until the pumpkin hits the scale is the thickness of the pumpkin. Other than one pumpkin I've never known for sure if a pumpkin was going light or heavy. One thing I've noticed is that pumpkins that tend to be very wide and not very tall tend to go lighter on the scale. I think this is because the weight of the wider pumpkins tends to smoosh out the pumpkin under its own weight which kind of gives a false circumference measurement. On my pumpkin you can see that with the thrown.
Initially my pumpkin was normally shaped but fairly early on the thrown formed because the pumpkin squished out that piece of it. It was suggested to me that either that means the pumpkin is very thick and that it smooshed it out because of all of the weight above it or the pumpkin is very thin walled and as a result it couldn't support its own weight and the thrown formed. Who knows. I like to think it was the former example however.