The first two tiers of placed traps affect twice the horizontal area that the old ones did. (The vertical volume remains the same for all of them.) It's about the width and depth of a fireball now.
The next two trap tiers are twice the (horizontal) size of that.
The last trap tier is twice the size of that, filling whatever room you're in with a pretty big plane of fiery (or whatever element you're using) horizontal doom.