It could've been called 'Not in a Big Tree' because it's not. But there is one there.
But it's called 'Not a Big Tree' because it's in a tree that's not big. Right by the big one.
An abandoned parking lot is available just south of the cache, room for lots of cachers. You can see the not so big tree from there.
Update 10/27/2023: The tree that was not big is now big, but not as big as the big tree it stands beside.