I agree that this does tilt more towards the long term implications and changes, and wrestlers generally need short-term changes to make weight.
If you are training and eating decently, the weight will come off, and the unused/unchallenged muscles will get smaller.
taking it to the extreme as seen in the pro images, well, that appears to also have been a byproduct of extra help.
loop it back to a guy like wiggans who made a dramatic change to body composition, it shows that weight is a huge function in the type of racing ability you could showcase. add 5kgs to him and he's not in the discussion.
as a matter of genes and potential, as with anything, some types have it easier in their build, musculature, composition, metabolism, etc. With a regular health check, keeping tabs on the juniors should not be so tough, especially if they have decent parents.