Can a retainer become too small for your teeth after a while? (Photo)

Hello, I had braces for 2 years and wore my retainer everyday for 6 months and every night for another 6 months like advised from my orthodontist. He was very firm about me wearing my retainer so I did what he said. This was back in 2014 and now I randomly wanted to try on my retainer and realised it no longer fits. My teeth are still straight but it feels like my teeth have grown bigger and are too big for my retainer. So I am just curious should I be concerned about this? What can this mean?

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