How can I get a cupid's bow without making my lips bigger? (Photo)

Thank you for your question. An excellent technique for lip augmentation is fat injections. It offers the only natural method for potential permanent results. I suggest that you move forward and consult with a board certified facial plastic surgeon.
Best wishes,

A small amount of filler to your cupids bow would help turn it up, without making your lips noticeably larger. 

When considering changing the shape of your lips it may be helpful to consider that  people who do face reading believe that the shape of your lips tells a lot about your personality.  
Moving on to your question without increasing the size of your lips significantly it is possible to increase the peak of your lips with some minimal placement of filler.  
Also if it is helpful, on line Cosmopolitan has  an article:  "What the shape of your lips says about you"
(this MD has no relationship with Cosmopolitan - found an interesting article on line0