I am writing to see if other agree that this is a bug. It's possible that this is what people have come to accept, but it seems totally wrong to me.
I have a 6" x 6" box that I have drawn guides to represent the boundaries (vertical at 0 and 6 and horizontal at 0 and 6) because I need to keep the contents in that are exactly the way that it is. I then expand my canvas to 200% x 200% and make sure that I have the center square selected. I am fully expecting to see 3" added to the top of my content and 3" added to the left with my guides in tact.
Instead, my guides don't move and the entire previous content is now centered in a new space that starts at previous 0,0 and extends 12" in both directions.
Doesn't it make more sense to keep the content positioned exactly where you have it and then extend the canvas relative to the position you mentioned. I now have to recalculare the 6" x 6" square that I had so delicately and deliberately setup and hope to reproduce it in this new location.
This seems like a bug to me, but I don't want to submit a bug report if this is what other people have come to expect. What good are guides if this is how things work? Is there a better work around than having to reestablish my guides? You would at least assume they would move the guides for you.
If you want your previous square centred in the new larger canvas with 3 inches added all round, make sure 'Relative' is not checked in the Canvas Size dialogue and ensure the anchor point is central and use 200%
If you only want 3 inches added to the left and top (and nothing added to the right or bottom), make sure relative is not checked in the Canvas size dialogue, put the anchor point bottom right, then use 150%