Ummm... There seems to be a problem with your Item Plugin.
It's not telling NSE that the sprite is compressed,
and as a result it's saving the sprites as uncompressed data.
The fix is easy.
When you read the Lz77 image data, output the DataLength to the sprite's "CompressedSprite" int.
byte[] ReadLz77Bytes(int Offset, out int DataLength)
Right now, CompressedSprite == -1;
Thus, NSE thinks the data is uncompressed.