I am also feeling like the market cap is increased too much so that none of the whale is able to manipulate the market price but still if some whales collectively want to manipulate the price then they may do as they will need to buy some bitcoins at lower price.
The market capt doesn't mean anything when it comes to whales being able to manipulate the price. Important to know is that the things that greatly influence the price are the exchange from where the whale wants to manipulate the market from, and the number of available (tradable) coins in the market (specifically in the exchange the whale operates from). The market cap of Bitcoin is a combined number of every coin in existence. At this point I don't even think 20% of the total number of produced coins are circulating in the market. Says enough.
The market cap is very easily manipulated because it doesn't relate to the actual liquidity of an exchange. People on an exchange can often change the market cap of a coin with just a few Bitcoin, sometimes even less, and the price changes very dramatically. Even Bitcoin exchanges have insanely low liquidity right now.