The changes in sabertooth aren't that unbelievable, you can always grow hair or muscle given enough time. He'd especially adapt quickly to environmental conditions and the first movie did show him trecking through snow. Also maybe if you spend enough time about Magneto his magnetic fields stimulate your genes or something?
I'm more interested in the huge shift in their relationship. Origins left much to the imagination of what happened in the movie universe between its end and the beginning of X1.
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