Question about the Law of Definite Proportions If we know that:

$1g$ of carbon + $1.33g$ of oxygen = carbon monoxide

$1g$ of carbon + $2.66g$ of oxygen = carbon dioxide

My question is: why is that in the second case where we doubled the amount of oxygen we don't get double the amount of the first case (2 times carbon monoxide) 

Or why don't we get the same amount of carbon monoxide and the remaining mass of the products is simply oxygen that didn't form any bonds.

Basically, what forces the reaction to occur in a certain way just because the proportions are different but the elements are the same.
In reality, there is not a sudden changeover of $\ce{CO}$ to $\ce{CO2}$ production when the oxygen supply is increased. This is the reason that all dwellings should have carbon monoxide detectors.

As an example, along with $\ce{CO2}$, gasoline (petrol) engines commonly produce 3-7% $\ce{CO}$ before additional air is injected.

The amounts you cite are how much oxygen is in pure samples of each gas, but producing either gas often requires filtering out traces of other gases that were also made in side reactions.

Producer gas is a fuel made by the incomplete oxidation of carbon (from coal or other source, sometimes with the addition of water) and contains both flammable $\ce{CO}$ and nonflammable $\ce{CO2}$ (and $\ce{N2}$ from the air supply). During fuel shortages, gasifiers were cobbled onto vehicles to enable them to run on this type of fuel.