"I am smaller than you."
Translation:Ik ben kleiner dan jij.
You might hear that sentence in Dutch but it's wrong. Since you are comparing the subject ("ik") to something you need to use the nominative form of you ("jij"). It's like the difference between "me" and "I". You could say "You are smaller than me" but if you make the sentence a bit longer you can see that using "me" doesn't work: "You are smaller than me am". In Dutch you should always use "ik" instead of "mij" (or "jij" instead of "jou") in this case.
As I explained to IgorHenriqueA: you have to be able to put a verb behind it in this case.
Since "jou bent" is not correct, you can only use "jij". You won't use "je" here because this is a "stressed/marked" situation. That is not to say that there are not people that say "jou" at the end, but those people are wrong to do so.
Many times 'je' and 'jij' are interchangable when used as a subject pronoun, although 'jij' puts more emphasis on the word "you."
Say, when your kid brings you a homemade necklace, you can ask -"Heb je dat gemaakt?" or, -"Heb jij dat gemaakt?"
The first question focuses on whether the necklace was made (versus say, bought, gifted, or stolen). By using 'jij,' you emphasize the maker, as if to say, "wow, I can't believe YOU made that!"
When 'you' is emphasized/stressed, like in this comparison sentence, you have to use 'jij.'
Also, if no verb is present it is almost always 'jij.' For instance;
Jij ook? = You too? Hé, jij daar! = Hey, you there! Jij en ik = You and I
However, when 'je' is used to replace 'jou' (i.e. as an object pronoun), you can never replace it with 'jij'. For example:
Ik haat je = ik haat jou = I hate you