Is there an explanation as to why different lightsaber crystals have different HP requirements? Aren't they all the same size? Why are some 1, some 2, and some 3?

Simplest answer is balance. Some have really good effects, so the HP requirement is a balancing element.

If you want a story reason? Well, it could be that that particular crystal requires more power than others, or is heavier so it requires better infrastructure, or brittle and needs extra reinforcement. Maybe certain ones need special kinds of emitters that produce the blade, due to differing mineralogical compounds within the crystal.

22 hours ago, kenngp said:

Is there an explanation as to why different lightsaber crystals have different HP requirements? Aren't they all the same size? Why are some 1, some 2, and some 3?

22 hours ago, Blackbird888 said:

Simplest answer is balance. Some have really good effects, so the HP requirement is a balancing element.

If you want a story reason? Well, it could be that that particular crystal requires more power than others, or is heavier so it requires better infrastructure, or brittle and needs extra reinforcement. Maybe certain ones need special kinds of emitters that produce the blade, due to differing mineralogical compounds within the crystal.

As @Blackbird888 said, there are several different reasons for the differences. Aside from the aforementioned ones does indeed include differences in size. Nothing in the rules nor canon state that every type of crystal is the same size.