Yeah, I've got some pretty serious buying power. But you have to realize that while the number is very big, it does not translate to trading with that much exposure, not even close.
The reason the firm needs to give me a lot is to enter opening orders. For example if I enter 120 stocks x 4000 shares each x $40 average share price x 2 (some firms count the buy and short order against bp, eventhough you can't be filled on both), that's $38M in bp needed just to enter the orders. Plus a guy needs wiggle room (more orders, more size, etc). It's more a way to get around the software so I don't have orders being rejected because I've run out of bp. On a real busy day, I might have $4 million in play with openings.
BTW, I only keep about $20k in this account. Prop bp is a great thing if you know how to put it to good use.