If you can bend your thumb, then the hole's too big. If the hole is big enough for your thumb at its largest (everyone's thumb swells and shrinks with time of the day, temperature, humidity, and other factors), then you need to add tape at the back or the front of the thumb hole, so your thumb just barely fits in the hole.
You can also use tape on your thumb or devices like Ron'c Magic Carpet or the ThumbBillow in the hole.
BUT the basics are if you can bend your thumb, the hole is too big. period. (Honestly)
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"Those who do not remember history are doomed to repeat it."