I think alot of it depends on the individual bird, it's bloodline and sex.
For example, there is a mahooosive difference in size between this pair, so the weight must be significantly different aswell. You can't see too well from the photo, but she is twice his size. At least a head height taller and twice as wide! She's bigger than the average peachfaced aswell.
http://www.agapornis-personatus.com/...titchandco.JPG
However, they are male and female so there's supposed to be a difference. Here is the same female with another female. Again, can't see the height difference as she has her head bowed down, but cant miss the how wide she is lol.
http://www.agapornis-personatus.com/...tchandblue.JPG
These are two males. Both are fully grown and feathered. The green edged male is a good head height bigger than the blue pied.
http://www.agapornis-personatus.com/...gedandbont.JPG
I seen some masked that are really small, like blackcheeked small, and I seen some so big they require 5mm rings you would normally use for long feathered peachfaced or abbysinians. At last years show there was a family line of wildcolour masked, which were huge. They were without a doubt the biggest masked I had ever seen. I bet they weighed twice the weight of my smalled mask lol.