I was making use of a 150 Mb HDR file for Environment Lighting. But I removed it before packing all other files into my .blend file. However, Blender is still telling me it cannot do it as it cannot find that HDR file… but I already deleted it!!!
Just to see what it would do, I used "“Menu File/External Data/Find missing files”, to point to the missing HDR file. Unfortunately it did pack it, now my .blend file is over 150 Mb Even though the HDR file along with its node are no longer there!
To delete it, I went into the Node Editor / World, and removed its node. Then replaced it for a nodes set not using an HDR file.