The items that give you little XP are there to boost your multiplier. Items that give you a lot of XP generally drop it. You're supposed to build up your multiplier with the small items and then use as many of the big items as you can at one time.

My only problems with the AI are that 1) I use Sazh, Hope, and Vanille, so they're always standing right next to each other, which is great for healing but not so great for avoiding strong spells, and 2) my Synergists insist on casting En-element on characters that never physically attack.