fat cat, big wig, top brass: important people. Fat cat is someone who is rich and has grown fat on his wealth. Big wig means an important official - in the past higher class people used to wear wigs - judges and barristers in England still do - the bigger the wig, the more important your position. Top brass means a leader - someone at the top of the pile who gives orders. It comes form the military - where brass refers to the insignia worn on the uniform.

small fry: Fry are baby fish. Small fry means someone very unimportant - a commoner - the same as everyone else. At the bottom of the pile.

the high life: The good life - expensive, luxurious, lots of fun.

A: "Things have changed since we were young. He's become a real fat cat, drives a limousine, drinks the best whiskey, hangs out with the big wigs, the top brass. I'm just a small fry compared to him. We don't see each other at all."
B: "But surely he hasn't changed that much. He never used to care a jot for money or status."
A: " That was when we were students. That's what you're supposed to think when you're a student. Things just change as you get older."
B: " But you haven't."
A: "I just never made it, that's all. To be honest, I wouldn't mind living the high life for a while."