okay i have 2 budgies and i don't now how old they are, if they are male or female and 1 of them is small maybe she didn't get enough food when she was being feed by her mother, she is really active as well could that be the problem she's eating though the day. And the other one is bigger, plus they are not tamed. They are less then a year old and our cage is big they are both different breeds could that make a difference?

It may just be that they are different types of budgies. English budgies are bigger than the wild types... What you're looking for if she's underweight is a very defined breastbone. You should be able to feel the bone, but there should also be 'padding' (muscle and fat) on either side of this, and you shouldn't be able to feel the ribs. Its a bit hard to describe without showing you, but if she's eating well and active, chances are she's fine, but if in doubt visit an avian vet

If you could post pictures we can help with sexing etc.

 

 

okay