I assume you mean their breasts, not their chests. First some reactions to other answers.  I would be careful about wider angle lenses as it makes them look less attractive in general. Poses and clothing can do a lot. And using a model with bigger breasts helps a lot, too. However, post editing is often applied in the professional world. In photoshop [liquify [LINK]][1] can be used, and some free softwares have similar tools. That type is most often seen used to thin the chins and waistlines of the models (there's a wide range of videos showing how to make a jane doe look like a "supermodel" - often used by women on facebook to fight low self-esteem).

I once made a video on how to use that kind of tool in the [opposite way [LINK]][2].
 
And used it to show that the pick for wonder woman isn't so bad as people claim (includes breast enlargement, but it is a bit roughly done - I made it for a nerd page as a joke):

![Making women more fullbodied][3]


  [1]: http://help.adobe.com/en_US/photoshop/cs/using/WSfd1234e1c4b69f30ea53e41001031ab64-75faa.html
  [2]: http://www.youtube.com/watch?v=pVSYrd1VJMQ
  [3]: https://i.sstatic.net/7rQwy.jpg