Autosaves aren't compressed - actual saves are. When Civ3 came-out, originally no saves were compressed, but people's save directories were getting huge. So after a patch, they started compressing them - but on low-end computers, that was taking a noticeable time - a pause at the start of every turn when an autosave was made.
So they changed that to only compress manual saves. that way, you don't get the delay each turn when autosaving, but the saves directory doens't get too large.