My house breaker box has a 150A main breaker.

There's a 240V / 50A breaker in there that leads to another breaker box outside, and it has a matching 50A breaker.

Now I need a second 240V / 40A breaker at that outside breaker box, meaning that I need at least 90A out there.

I obviously can't just run bigger wire and install a 100A breaker at the main box, that would only leave 50A to run my entire house!

What are my options?