What is the science behind magnesium reacting with different concentrations of hydrochloric acid?

I need to know a scientific explanation of this for my GCSE science coursework. I really i have no idea what happens, please help!!! In my experient i have produced a set of results with 60% acid and 40% water etc... I am measuring the amount of gas that is collected at different times during a 30 second period. I also need to know why the concentration effects the results and why the size of the magnesium must stay the same. Thank you :)