What causes heart palpitations?

Generally the left side of the heart is bigger than the right.

A palpitation is when someone is aware of their own heart beating. That's quite common and I'm often aware of my own heart beating specially when I'm in bed going to sleep, but being aware of your heart beating is not necessarily abnormal. Lots of things can cause palpitations, but the times when it requires investigation is when someone is aware of their heart beating and feel very unwell, or when their heart beats quickly for a long time or they feel light headed. Sometimes it can be normal and sometimes it requires further investigation.