Somewhat.
It's true that the filesystem on MacOS X (HFS+) does do some defragging on the fly. Files under 20MB, when they're written, get put in a new contiguous block if at all possible. (If it's not possible, you've got problems already.)
A straight copy though, to a fresh drive, *should* be fairly contiguous to begin with, but if it's a drive that's already got stuff on it, then yes, your guess is probably correct.