All the above weights are within the normal range. I raised parrotlets for a few years, more than a decade ago. At that time 30 grams was common and many weighed more. As parrotlets became more common and people began to breed more carelessly (not selecting their breeder birds for health, weight and character) the average weight began to drop.
I had one bird who came from an old time breeder, one of the first to breed and show parrotlets in the US, and he was a whooping 46 grams. He was the biggest that I ever came across. That bird could sure eat. He was actually over weight though.
To assess the body condition of a bird you feel the keel bone. If it is not protruding and is somewhat padded but still easy to find then they are usually a good weight. A overweight bird will have fat covering its breast and it will be difficult to feel the keel (breastbone), while and underweight birds will likely have a protruding or sharp keel bone.
The personal weight history of an individual bird is a great thing to have to compare his historical weight to his current weight so you can be alerted to possible health issues which might be indicated by the change in weight.