Poker chip shrink?

I bought some poker chip set from amazon to upgrade my game, and I though I got some decent ones . These chip feel nice, and its heavy weight allow you to stack them pretty high without them falling off by the slightest touch. I store most of them away for boardgame night, however I keep a stack of 20 on my table to fidget with.

A year later, all the chip still look nice, however, the I notice chips I keep by my table has shrunk by 5%. Putting a stack of 20 of the chip on my table next to a 20 stack from the stored chip, the stack of chip on my table is 1 chip shorter than the stored one. This is a huge issue since height comparison is the fastest way to count chip stack.

Is this a common issue with all type of chip, or does my chips just have poor quality.