"Surely, in the WAS/SEA example, it doesn't mean you bet 700 to win 100!"
I'm not sure, but maybe it just means that. According to this
"... The difference between the two will continue to climb as the disparity between the favorite and the underdog increases. In a boxing match it would not be unusual to see odds such as:
Joe Louis -700
Ray Leonard +550
In this instance, those betting on Joe Louis are being asked to risk $700 to win $100, while Ray Leonard backers are risking $100 to win $550. ..."
I know that in your example it's not box but basketball, but... Who knows, maybe the difference between the teams is such big.