For Show Flemish Giant does have to weigh 14 pounds. HOWEVER there are so many things you need to take into consideration about the weight of a Flemish Giant.
1.) Breeding, did this doe come from a show line? From a nationally recognized breeder? If so then it is in her genetics to be bigger. If she came from a little backyard rabbitry she probably won't get over 15 pounds
2.) Feed - Quality diet means they will grow to be big and strong!
3.) Bone - yep bone, the bigger bone they have the bigger their frame is, so the more muscle they will have. If she is a small petite dainty little doe, she won't be very big. However, if she has big thick legs, and wide shoulders, and a wide backend, she will probably be up to 20 pounds
As far as the 34 pound thing, that is the rabbit on record. They won't get over 28. That is the biggest weight recorded here in the US.
Now Heres the important part.
Just because flemish Giants can be 20 pounds DOES NOT MEAN THEY SHOULD ALWAYS WEIGH 20 POUNDS. Repeat that sentance, write it down 100 times, anything you can do so you will remember it. I can't tell you the number of people that I meet that have a Flemish Giant who should only be 15 pounds, as in thats how much they should weigh for their size, anything more than that would make them overweight, and they want to know how they can get their rabbit bigger. Their are many different sizes of Flemish Giants. The difficult part is trying to figure out what your rabbit should weigh. If you have a pedigree, average out the different weights and the average weight is probably what she should weigh.
If you do not have a pedigree, you can go by what she weighs at about 1-1 1/2 years of age. Flemish Giants can continue to grow until that age, so assuming she isn't already obese because of diet at one year of age that weight is what she should always be at.
Sorry for the lecture :soapbox and rant. its just very important because Overweight Flemish Giants can have So many health problems, and it would be easy to prevent if so many people didn't try to make their Flemish Bigger than what it already is. I mean really, isn't 14 pounds big enough