Vine Ogres, worthwhile?

[Shangrila00]

Hmm, MA Mictlan does have good use for decoys against fire/attack large, what with their size 4 couatls and no national unit larger than size 2. But while they have double the hp, vine ogres are only size 3. Spells should definitely get drawn in by the much higher hp concentration, but does fire large target based on hp or just size?