There are a couple of factors at play here.
1) The higher the level of the mob, the more SP you'll get, up to 10 levels above you. So if you're rank 25, and you fight a rank 15 dodo, you'll get less SP than if you fight a rank 25 dodo, which will give less SP than if you fight a rank 35 dodo. But if you fight a rank 50 dodo, you'll still get the same SP as if you fought a rank 35 dodo.
2) Different mobs give different base amounts of SP. Some mobs -- like raptors and ahrimans -- give more SP (base of 150 SP). Some mobs -- like chigoes, coblyns, and puks -- give less base SP (base of 63 SP).
3) Your party size impacts the amount of SP you get. If you solo or duo, you'll get 100% of the SP per mob. If you trio, as long as everyone in your party is within +5/-10 levels of you, you'll get 90% of the SP per mob. With 4 people in your party, as long as everyone is withing +5/-10, you'll get 85%. Etc. While that sounds like they're punishing you for partying, you kill mobs so much faster and easier that you actually get much more SP in a party.
4) At lower levels, you get a bonus to the amount of SP you get per mob. Up through level 23, they take the SP you get from the mob, and they multiply it by some number. Between levels 1-6, they multiply it by 4.0. That number drops until you get to level 24, when you get the base SP (in other words, they multiply the SP by 1.0). So you're probably getting more SP from your lower level jobs because they're under r24, so they're getting an SP bonus per mob.
If you want to read about how SP works, check out Kanican's livejournal.
But the best advice is usually to get into a party. Fight mobs that are 10 levels higher than you. Grind camps -- especially at beastman encampments -- now seem to give the best SP.
It's both speed and difficulty. You get a bonus when you complete the leve based on 1) the leve difficulty (how many stars you run it on); and 2) how quickly you complete the leve.