Transferring items from one storage component to a different, larger component significantly increases how much storage gets used

[Xyvyrianeth]

Modpack: MC Eternal v1.3.7.1 (Minecraft v1.12.2)
Mod version: 2.6.5

I use an ME system to store materials harvested from an automated Mystical Agriculture farm. Decided today to swap out my 256k storage component with a 16m storage component because it was getting full (257135 bytes used). Did the math on all the materials needed before crafting anything, put it all together in a few crates (needs over 15000 redstone), and got down to business. 2 hours later, I've got a 16m storage component. I turn off the autofarm, put the empty 16m in the ME Drive, and the old 256k in a connected ME IO Port to transfer everything.

Fast forward about 6 minutes and the 16m storage component now has 7740522 of 16777216 item bytes used.

Over seven million. Almost eight.

Why is there a ~2900% increase in byte quantity when the actual content being stored hasn't changed?