How does a creature's size scale - backwards?

[Totema]

Hi all,

I'm reworking a creature for my 3.5 campaign - specifically, I'm trying to bring down an Assassin Vine from CR3 to CR2 - but the adjustments to size and natural armor are perplexing me. This wouldn't be a problem if I was raising its HD; I can plainly see that at 5 HD the assassin vine would increase in size and thus gain additional natural armor, but the MM isn't very clear on what happens if I take away some HD instead. Could anyone point me in the right direction?

[TuggyNE]

I'm not aware of any formal guidance, so you're mostly on your own. However, consider adapting PF's Young template for this sort of thing.

[Debihuman]

There are two ways to do this. You can merely remove a HD to scale it down or you can scale it down by size . They do different things and you then figure out the CR. See improving monsters by size here (http://www.d20srd.org/srd/improvingMonsters.htm#sizeIncreases) and then do the reverse.

Going from Large to Medium give you: -8 to Str, -2 to Dex, -4 Con, -2 natural armor and +1 AC/Attack.

Debby