Bitcoins price is based on supply and demand on the market.More demand higher price more supply down price.Yes you can manipulate bitcoins price if you hold a huge supply or supply that may be enough to sell or buy in the current market status
I do not think that there are many people who hold a large number of bitcoins in order to manipulate the price. But this assumption can not be ignored.
And there are like 10 exchanges, where buy and sell orders combine over 1000 bitcoins and hence its not easy to change the price of bitcoins. There are certain whales whom I see on dice sites winning 10-20 bitcoins and they say that they changed it to fiat yet I never noticed a significant change in price, this itself means that the market cap is enormous and can't be manipulated easily.