If the car was running it could be catastrophic to the engine. (Broken pistons bend connecting rods, corrosion in internal places) So the engine is one thing now let’s look at the wiring harness the seals for the connectors are good but not waterproof so the car may have electrical faults that are hard to track down. And all the mold in the air ducts, carpet, seat foam. Who knows. What kind of car ?

If you up size it will use less of it’s overall capacity therefore …in theory it will last longer. Less wear and tear less heat, I’ve done this sizing aircraft generators

Assuming same brand and quality