My purebred golden is 48 pounds at almost 10 months. She was probably about 40 at 8 months though (just had another growth spurt). Breed standard for golden girls is 55-65 pounds...you'd be surprised at how tiny a 55 pound golden looks. Nearly all goldens and labs you see are overweight, so you don't realize how many small ones you might actually see that just look a lot bigger because they're overweight.
Also, both goldens and labs take a long time to reach their final weight. In fact, it's dangerous to grow too fast (they can be more prone to hip/joint problems). For goldens, they should reach their final adult height by about a year old, but they don't reach their final (ideal) adult weight until 2 years old. In other words, they grow up all the way and then fill out over time. So you can expect a golden to add another 5-10 pounds after they are a year old. I don't know much about labs, but I am guessing they are about the same.
Puppies go through growth spurts too, where they don't seem to grow for some time and then all of a sudden just shoot up (kind of like teenagers). I would not think by any means that your 8 month old is done growing. Sure, she will be a smaller dog, but I'd guess she's got another 10 pounds, maybe more in her.
And to add to the mix stories, I know of lots of lab/poodle mixes who people think should be "standard" around 60 pounds because the poodle was 55 and the lab was 65. Yet these dogs inherit the height of the poodle and the bulk of the lab and weigh 60 pounds at 6 months old!!
I'd be glad that your baby is a bit smaller (as it seems you are), and as long as she's healthy (not too skinny), just let her be and see how much she grows!!