Answer:
If your muscle is too beefy, the muscle can restrict the joints because there may be a lack of flexibility. It's hard for body builders to move without struggling because they have so much muscle mass, and it's unbalanced.
I suppose it's along the same lines as when an obese person. There's too much of other things preventing your full range of motion.
I hope this helps. Pls let me know if it doesn't :)