How to "jump" while riding my bike?

[Vladimir Kozlov (ECW)]

I know I need practice but how and do I need bigger muscle to carry myself? I'm thin and small and I'm wondering why I can't do it.

 

[M V]

Presuming you are talking about a "bunny-hop", its key to commit fully and rapidly to each part of the hop. Also, tire pressure is very important; you need a low enough pressure to get good compression, yet high enough pressure to get good rebound. Experiment with tire pressure to find the range that works best for your weight and riding style.