If you have 500 bhp in a car that weights 1000 kg, you have 0.5 bhp per kg. The power-to-weight ratio is 1:2, because the weight is twice as big as the power.
A ratio of 1:1 is when you have 1 bhp per kg (like, 700 bhp in a car that weighs 700 kg).
It's the same with gears: If you have one gear with a teeth count of 16 and another gear with a teeth count of 24, the gear ratio is 16:24, which can be simplified to 2:3, or more commonly 1:1.5