It would be large over small, but usually alpha doesn't relate to size but priority, that it is the most important. I'm am not familiar with this particular phrase but I hear alpha dog often which refers to a guy who is is the coolest, biggest, most manly, gets all the ladies, a big ego, and so on in a group. Hope it helps
In this context, it is a technical term used in the financial market, it is a technical measure of an additional rate of return on investment because of the additional risk of losing money. A high alpha means that you have a greater chance of making more money, but also with a greater chance of losing money.