I should also add that advantages of working for a big pharma are the benefits you get - bonus, health insurance (although small companies do this too), being treated out, pension scheme, life insurance etc.
A lot of big companies do this, not just in pharma. It's cheaper than paying out big salaries.
Disadvantages of working for a big pharma include, you could end up being stuck in the same project for years and years. However it would be much less stressful (and potentially boring).