The reason why it was a beam is because he put more energy into it.
Most wave attacks are actually ki beam attacks with a grip of energy in it, so that its a bigass constant trail (with the ball...) . The more energy, the bigger the attack.
The kameahameahamaahahaeahaheae (I can't spell that thing),
when used by goku, is a ball with a small trail, but when he powers up and uses it (like when he shot cell from below in the cell saga), it is a giant trail with a big ball at the end, you just dont see the ball (accept for when cell shot his attack at goku, and goku was being chased by it into space, but used the instant transmission, blah blah blah).
That's what the big bang attack is. Infact, I really don't remember seeing him just shoot the big bang ball.... I thought..... nevermind, I'm too lazy to type..