What factors determine the power curve of a jet engine?
Not a direct answer, but one thing to keep in mind is that thrust has very non-linear dependency on RPM. 70% N1 may mean 20-30% thrust. (And that's static thrust; in flight the difference may be even greater).

I don't have exact figures for AI-25, but for a similar engine I remember the gradient near 100% was around 5 (5% thrust for each 1% N1).

Actually, I have some data for Williams FJ33, which is comparable.

On the ground, it idles at 25% N1, which is only 3% of thrust. For 50% N1, you still have less than 20% of thrust. 60% N1 -> ~28%, and for 98% N1 you have over 100% of static thrust.

By the way, AI-25 on L-39 is 'overrated' compared to Yak-40 and its N1 goes to 107%, if I remember correctly. So it's all not that bad.