7500 KW generator and a 30 amp line
There's nothing wrong with overdesign.   Tearing out 100A-rated cable simply because it's "too big" is just wrong.  I'm not saying you should oversize every wire, but if you are in an situation where the existing wire is larger than needed, it works in your favor.  

However there is something wrong with overfusing.  Fuses and breakers protect wiring and equipment.  A breaker which is too large will not protect the equipment.  That's not a worry if the generator has its own breaker onboard.  

If the generator has its own fuse or breaker protection that is correct to protect the generator, then the input breaker in the panel is allowed to be larger than the generator.  The original setup was fine.