Don't forget that OWD is a huge factor in this. Even if the bey is quite heavy, if it's all in the center it'll make the bey's stamina very bad. But if the weight is compressed into the center it also makes the bey good at staying around the center.
When a bey becomes EXTREMELY heavy, even the slightest of unbalance unbalanced the whole combo. Extreme weight also causes less centrifugal force so it becomes harder for the bey to become upright again.
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