I had some trouble with this- both of my little fluffbutts used to be overweight. It’s important that you start weighing them, and I would suggest seeing your avian vet for a checkup. The vet really helped me as my biggest problem was fixing their diet up, as mine were seed monsters. It took 8 months for me to convert them to pellets. One of my budgies is also on the lazy side, despite my best efforts with giving her a variety of toys and encouraging her to play games that require her to fly, she is a little love bug and just wants to sit on her humans shoulders! To get her moving, foraging for her seeds really works . Foraging is great as not only does it make them work for their food, it is good intellectually and mental health wise .
There is a lot to take in, a vet is the best placed person to give you advice specific to your birds needs. Also, I will say, I need to be mindful with my hen- if I lay off her weigh in and diet monitoring her weight creeps up again. I have to stay on it!