Well, it does say "Resolve these attempts as normal, except that they don’t provoke attacks of opportunity, you use your caster level in place of your base attack bonus (for disarm and grapple), you use your Intelligence modifier (if a wizard) or Charisma modifier (if a sorcerer) in place of your Strength or Dexterity modifier, and a failed attempt doesn’t allow a reactive attempt by the target (such as for disarm or trip)" in the SRD version of the spell under the combat maneuvers sub-heading, and other than the "Resolve these attempts as normal," it doesn't mention Improved (Grapple/Trip/Bull Rush/Disarm) nor size, so as written, those modifiers apply "as normal" the way the spell is written (in the emminantely quoteable SRD, at least).
But yes, it does seem kinda wierd.