Are DC-DC converters designed with fixed output or fixed output/input ratio?
The search term for what you are describing is "proportional dc-dc converter". For example, you can buy high-voltage DC-DC converters, where you put in 5-12V and get 1000 times the voltage out (5K to 12K volts). But for most DC-DC converter uses, the purpose is to get a fixed output voltage even if the input varies.