It depends on the number of celestial bodies and the time period it covers.As I do not have any idea of the SPICE-library and it's inner workings, is it possible to give a rough ratio of the size differences?
Something like "basic" 10 MB vs. "max-fidelity" 300 MB ?!
I currently use the orbiter.bsp file, which is approximately 600MB in size, as described here:
The major roadblock to OpenOrbiter 64bit stability is a lack of source code for some of the celestial bodies (mostly moons of Uranus and Mars). In past Orbiter releases, these were always packaged as just a DLL and I'm not sure Dr. Martin S. ever had the source, at least it was never added to...