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The implied explanation is that the characters are "growing up" from young teens into adults as the series progresses. Still waaaay over the top.
The series IS still going, with over 700 manga chapters and nearly 600 anime episodes, along with over 10 movies. Doesn't look like Oda plans to stop anytime soon, either.
CG models are even trickier to work with than traditional animation, in most cases (in my opinion).