aldehydes are easily oxidised but ketones are not
In the oxidation of aldehyde, the hydrogen attached to carbonyl compound is replaced by an $\ce{-OH}$ group resulting in the formation of carboxylic compound. In case of ketones, bulkier alkyl groups ($\ce{-R}$) should be replaced by an $\ce{-OH}$ group to form carboxylic compound. 

We know that hydrogen abstraction is much easier than that of bulkier alkyl groups. Likewise, after the oxidation of the aldehyde, the carbon chain containing the $\ce{C=O}$ group remains same but due to loss of $\ce{-R}$ group during oxidation, the length of the carbon chain decreases in ketones.