Are big miners a threat for Bitcoin? According to this graph, the biggest miner is so big that combined with two of the three runners-up they would have more than 50% of the hashing power.

Is that to be considered a threat to the bitcoin network?
As others have mentioned, those are mining pools, not miners.

Large mining pools provide the benefit of allowing small miners to earn more regular income, which encourages participation in bitcoin mining and strengthens the bitcoin network. 

They are also a threat because they can be hijacked or DDOSed by a party wishing to do a >50% attack. 

A temporary >50% attacking from a large pool being taken over by a hacker or DDOSed is a much smaller threat to bitcoin than a party acquiring more than 50% of the network hash-rate through their own hardware and being able to conduct a permanent/long-term >50% attack, so mining pools, by increasing the contribution of honest miners to the network hashing rate, benefit bitcoin more than they threaten it.

Features could theoretically be incorporated into mining software to reduce the threat of hijacking and DDOS attacks too, like automatically switching a mining machine to a different pool if the mining software detects previously confirmed blocks becoming unconfirmed or the mining pool becomes unavailable, respectively.