Bilic has revealed that if the France international attacking midfielder does not train regularly, then he puts on weight.
“Players are sometimes good but arrogant,” The Mirror quotes Bilic as saying about Payet. “It is not the case with him.
“He is calm and loves to train. In fact, when he stops, he puts on weight - I have told him. And then he is no longer the same.
“He has to be in perfect condition and we saw that against Romania. It is for that reason he can get there have 10 centimetres before to play the ball. To find this space, you have to be fit.”
It is quite a stunning revelation from Bilic, but West Ham fans should not be worried.
Payet is a brilliant footballer and was one of the best players in the Premier League this past season.
The 29-year-old is playing for France at the Euro 2016 finals at the moment, and he could end up winning the competition.
While there is a chance that the former Marseille star could put on some weight when he goes on holiday after the European Championship, he is certainly going to get back into shape at the start of the 2016-17 campaign.
West Ham will play at the Olympic Stadium in front of over 50,000 fans next season.
Payet will be a key player for the Hammers and will be hoping to lead the London outfit to a top-six finish.
The Frenchman scored nine goals and created 116 chances in 30 Premier League appearances for West Ham this past season.