No, it isn't a case of direct scaling to level. There may be a formula deep within the system, but it would take a lot of work to find...
This is one of the aspects I love about this game.
For example, in the Lifetap spell chain we have the following
1. Lifetap - 2 damage siphon
2. Lifespike - 1d10+1 damage siphon
4. Lifedraw - 6d6 damage siphon
6. Lifeleech - 7d10x2 damage siphon
7. Siphon Life - 8d6 damage siphon
10. Spirit Tap - 8d10 damage siphon
11. Drain Spirit - 6d10x2 damage siphon
12. Drain Soul - (7d10+2)x2 damage siphon
The oddball here is obviously the level 6 lifeleech, being far more damaging then even the level 10 Spirit Tap and somewhat better than the level 11 Drain Spirit (except for save DC). The big difference is the mana costs.
1. Lifetap - 2 mana
2. Lifespike - 3 mana
4. Lifedraw - 10 mana
6. Lifeleech - 53 mana
7. Siphon Life - 12 mana
10. Spirit Tap - 24 mana
11. Drain Spirit - 32 mana
12. Drain Soul - 38 mana
So the anomaly of Lifeleech's high damage is countered by an anomolously high casting cost in mana. A higher level spellcaster will discard lifeleech in exchange for Drain Spirit or Drain Soul.
This is what I like about the system... you can get devastating spells at lower levels at a commensurate cost in mana. This would be like getting a level 4 spell in D&D that, when cast, also used an additional 6 or 10 levels of spell slots.