If there are any electricians out there I would appreciate some advice.

My dad and brother-in-law are jacks of all trades of sorts.

They were putting in a new main breaker box at a house and decided to install a 200 amp box instead of a 100 amp box that was already in place. Everything went smooth and everything seems to be working fine.

However, I always thought that changing your house service from 100 amp to 200 amp isn't as simple as just changing the box. I figured there had to be some additional outside wiring down to bring the extra 100 amps through the service wire.

Am I wrong?

The service line that comes down the front of your house is what determines how many amps you can pull. If your main circuit breaker is 200 amp and your service line is only rated for 100 amp you have a fire hazzard. You can either replace the service line or replace the 200amp main circuit breaker with a 100amp one.

Right. You would be okay if it were the other way around, i.e. 200 amp service cable running into a 100 amp box. But not the way you have it.

Remember, the breaker is there to protect the wire. If the amperage capacity of the wire is less than the tripping point of the breaker, you could have a meltdown well before the breaker trips.