I don't think it shortens the lifespan.
I did not try to grow it fast - I've got some Cal Kings that hatched at the same time that are under 100g.
It's just a difference in growth rate - it grew faster and therefore was offered larger prey as it grew.
If I had offered it more food with the intention of growing it fast it probably would be bigger.
I don't think it has any impact on lifespan.
Power feeding, where animals are tricked into eating more (by manually inserting rodents into the the mouth when it is just about finished with the first) is IMHO bad and something I will never do.
I rarely offered him more than two rodents at a time - and those cases were just garbage disposal purposes and he had the option to refuse them (which he sometimes did). He's currently eating two large f/t mice a feeding.
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I decided my old sig was too big.