Bowser never uses "magic" to grow. Because he is a unique species of Koopa, he keeps growing with age until he eventually esplodes. The only exception I can think of is in Paper Mario, when you see him gett really large because of the electro-steroid device being operated by Kammy Koopa.
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"Why bother with real questions; we've got fantasy to debate over." Hey, I think I used that one already somewhere... oh well.
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TAYL: Talk At Ya Later
Lazer Mario