Read my previous post. Weigh is not an issue if the 150 pounder is trained. If its just brute strength and both are unskilled fighters, Id usually give it to the stronger guy.
With the untrained strength varies greatly and has less correlation with size. A big guy isn't necessarily strong.
There's also other factors involved like speed, endurance, overall athleticism, natural punching power, chin and gameness.
Not as easy as saying "bigger guy wins"... like you believe.