Honey, you should know not all women like muscles.
But on the other hand, there are lot of women are attracted to muscular guys. Here are some reasons:
Security: As silly as it may sound, men with muscle make women feel more secure. Women feel like their partner can protect them, no matter what. You might think that you can protect yourself, but this isn't something you're thinking about with your conscious mind.
Trophy boyfriend: Women are competitive creatures and having a good looking boyfriend really gives them an advantage. Some make it their goal to make their girlfriends envious. You might not admit it, but most probably you are doing it too.
Good Providers: Over the years numerous societies have considered men the providers, who needed to care for the needs of their family. Although this doesn't apply to modern day society, the idea is still present in the female psyche, making muscle men seem more attractive than the skinny ones.