While they can and will eat meals much larger than themselves in the wild, in captivity given the abundance of food, it isn't always a good thing to gorge them every week. In the wild BPs are literally opportunistic feeders and will attempt to eat anything they can overpower. The 10-15% guideline allows for a nice consistent growth rate without taxing their system too much.
While 11-12 gram mice are slightly smaller for a 120g BP, feeding one wouldn't hurt until you can get the appropriate size prey item.
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