Tapa and Tio (Jessenia) took on a very overweight budgie from a budgie breeder - im not sure the reason for the weight gain but with persistence she helped get him to a healthy weight again here is the link if you are interested: http://www.talkbudgies.com/showthread.php?t=73561
Now im not sure if it varies for pet type and show type, but i believe the healthy weight range for a budgie is 30-40g. You need a set of kitchen scales (preferably electric) for weighing your budgie. If you cannot get them to willingly step on the scales, you can put them in a box and weigh them (then subtract the weight of the box).
Obeisity can lead to a lot of health problems, just like in people - fatty tumours, fatty liver, decreased organ function, etc. So it is best to keep them at a healthy weight as far as possible
I believe 2 teaspoons of seed a day (plus fruit and veggies) is as much as a budgie needs daily as well as exercise, out of cage time of course