I hope things are improving for you and the bird since you wrote this. After flying a couple hundred birds for education and falconry I have a different take on it. Weight is unlikely the issue. Reducing weight is the last thing to consider in your situation. First off, check out the International Association of Avian Trainers and Educators. A great group with some very talented and educated trainers using scientifically sound beahvior and training priciples. I can give you some resources if you message me.

Most important fact here - he is not trying to outsmart you. He is simply doing what he has been taught to do. It is usually our training that is at fault, not the bird's weight, attiutude, intelligence (which is a construct). With your isssue of not returning to the fist. There are three areas in which a training problem could exist (or lilkely a combination of any of the three). The first is the message the bird is getting to fly in confusing, maybe your cues are not really conditioned that well, there are physical obstacles not present inside (things that may it their wing or tail), or other perceived dangers. This is known as the antecedent arrangement, making sure the environment is set up for the behavior to occur easily so that it can be reinforced, rather than failing and having to be punished.

It is easily plausible that being out in a tree or perch is just simply more reinforcing than being inside. In this case I often use the opportunity to go back out to a perch or tree as the reward for flying to the fist. Get a food treat and get to immediately go back out. It helps build confidence with the bird and trust with you.

Most often I see that the bird finds something punishing about flying to the glove. Hands are horrible perches, we move them a bunch without realizing it so they are unbalanced. Hold your hand perfectly stil and slowly bring them in to your body (or not if they don't like being close to you). They may hit their wing tip on our body (which is why I fly them to my hand in front of my body instead of to the side). Too often flying to the hand is the end of the fun, they get restrained and put in the box. So go back to me previous paragraph to help with that. Depending on your situation you may just train to fly to static perches and then to an exit crate. Many trainers have found much more success once the eliminate the hand from the routine.

I have inadvertenly trained some wonderful, undesirable behaviors in my time, which is why learning the science of training and behavior has been so beneficial in working with animals professionally and in falconry. But I hardly ever reduce weight. Focusing on training techniques and nuances is way more effective and ethically sound.

Good luck,
Andy Hall